Essentially, I think you can boil this down to one of two things. These are:
1) Your team is stronger than an army.
2) Your team has an opportunity that the army didn't/doesn't.
The first choice is rarely chosen, but that's not to say it isn't done. Something like God of War where the main character is portrayed as an unstoppable force compared to the 'regulars' within the army.
The second choice I think is the norm. But it is obviously very wide since essentially it boils down to 'everything else'. Whether it is stealth, mobility, a personal relationship, exploiting a weakness, or dumb luck.
There is a difference however, between reasons for "why the team succeeded" and "why the team was sent." Taking from the above post:
Only people free for the mission, e.g. your faction faces war on multiple fronts
This isn't a reason for why the team would succeed. This is just a reason for why they were sent instead of the army.
Technological restrictions (both for getting to the enemy and access to prototype weapons)
This could be either case, if you say "Only three people can go through this portal to fight the enemy." Then it is again, just a reason for why the three were sent. If the portal allows the team to get in to a weak spot, then it is a case being made for why they were able to succeed when other people failed.