The characters in the New Mutants had been around long enough that pushing them as X-Men Jr./Marvel's version of Teen Titans was getting stale. Plus, it was the gritty early '90s.

Ostensibly X-Force was designed/marketed as a "pro-active" team. So, instead of saving NYC from a rampaging Brotherhood of Evil Mutants, they sought out threats and neurtalized them.

In the mid to late '00s they were re-launched as essentially a covert execution squad that did all the messy work that the X-Men couldn't be associated with.