Some sort of cataclysm suddenly creates a territory full of deadly monsters and other dangers, but also valuable resources.

Trivial example: a magic volcano suddenly erupts and the mountain is now sprinkled with enchanted crystals and very angry elementals.

Normally, I believe, the majority of the exploration of such territory would be done by large teams of well-equipped professionals that were hired by governments, corporations, rich private individuals, etc.

What could make large teams ineffective, while encouraging lone adventurers and small groups?

Lord of the Rings is one of the works which explain this: if you want to pass unnoticed, the lesser the better.

A large party requires adequate logistic, preparation and support. Not ideal if you want to stay below the radar.

A small team, or even a single person, can more easily operate.

If your monsters have the capability to detect large vehicles and groups, this will make smaller groups more viable.

Think of a Gold Rush.

Sane professionals will proceed slowly. Clear an area on the edge of the territory, exploit it, keep it cleared, rinse and repeat.

At the same time, the desperate and foolhardy go in deeper. Go in, dart around, grab something, run away. These people are not patient and organized enough to "play it safe." They want or need instant gratification, no need to share the riches they will surely find next time, for certain. They feel it in their veins, their luck will turn if they go there one more time. Why, Smelly Jim got himself a crystal to buy his own castle.

Read about Klondike, Nome, California.

If the terrain is inhospitable, a large force can be a major drawback

As the romans found out

If the people know the land, they can set all sorts of traps which will make your numbers count against you, making it impossible to split up, difficult to coordinate, and easily picked off by flinging thins into the crowd.

Perhaps human beings emit some sort of 'radiation', perhaps magical of origin that draws monsters towards them. One person or small groups, fewer than 10 or so might have limited problems as their energy signature is too difficult to make out from background radiation, but the moment really large groups try their hands at entering the lands they can expect to become monster magnets.

For example: Something like the grimm from RWBY. The grimm are attracted to negative emotions, the more negative emotions there are, the more grimm will come. More Grimm means more negative emotions (anger, fear,...) and the vicious cycle has begun. The main problem the people from Remnant have with Grimm is that once such a cycle starts, it's pretty difficult to break it on time.

You will need some sort of reason though for how monsters survive. The grimm are pretty simple. They exist out of darkness, so they don't need sleep, food or water. Only shelter, because in one of the World of Remnant videos, the one about Atlas we see Grimm freezing to death. Your elementals will have no problem, but if you want an actual living and breathing T-rex or something, you will need to explain how a T-rex gets sufficient food. Of course all these creatures could just feed off of the magical radiation of the land or something like that.

I hope this helps.

For your trivial example, geography would be an important enough factor on it's own. Maybe those mountain routes are too narrow in some places for large groups to keep fast pace. The people up ahead need to wait for the people down behind to catch up. A longer column would take more time to cross the same ground than a shorter column.

This goes for any other scenarios where there would be such "bottlenecks". For instance, a shallow river boat can only carry so many people at any one time. A cave with a few narrow passages can only fit so many people before they started getting in each others' ways. A frozen lake can only support the weight of so many people at the same time.

Food and distance; any long expedition must eventually live off the land rather than supplies they bring with them. So if the area of devastation is wide, meaning that expeditions must travel far from the last base of supply, and edibles within the area relatively rare large caravans can't access the gem fields. They can't haul enough food to get there before they eat it all and neither can large parties, only small groups can live off the land and get in and out fast enough to succeed. This can also be done by restricting access to drinking water, which is often polluted by volcanic gases or chemicals leeched from a volcanic ash layer.

To a large degree this will be a temporary situation if the fields appear to be a longterm profitable venture. People will build infrastructure to exploit the site if it's worth it.

Laws or Diplomacy

Laws could restrict major corporations and state forces from entering the area. Maybe the surrounding states fear magical contamination from artifacts of the 'zone' or hold an anti-individualist credo that make them wary of the social disruption caused by returning 'nouveau rich' adventurers.

Alternatively, if more than one state borders on the territory they could come to an agreement not to exploit it. Perhaps the sudden potential of the 'zone' brought them to the brink of war and they agreed to stay out in order not to spark conflict.

In either scenario the territory would be avoided by state forces and 'respectable' businessmen who needed to work within the law. Instead exploration would be spearheaded by marginalised down and outs or by shady business concerns unconcerned with legality. Depending how seriously the laws/treaties were maintained you might also see plausibly deniable shadow actors such as privateers and government-backed mercenaries taking a cut for the governments that publicly banned exploration.

The monsters are dangerous enough that numbers don't significantly increase the odds of a party's success.

If a fire elemental can kill a small army just as easily as it can kill 1 man, then the smart strategy is to split up into very small groups that you don't invest much backing into individually. If you send in 50 small groups knowing that only 40 will come back, you can form a business plan around using the survivors' profits to replace your losses, and go again. If you instead send in one really big group and it gets wiped, you lose your whole business.