Finn had nowhere to go as a character because he wasn't based on a classic archetype like the original SW heroes were. He had no pre-destined journey that could be modernized, and he had no "symbolism" to which they could hang cool stuff on (like Han Solo had that great gunslinger/pirate angle to his character).
He just became a useless "guy with a good heart" that neither director seemed to know what to do with.
The idea of an "ex-stromtrooper" was a good start for something... but it seems that was all they had for him, and even that backstory never really amounted to much more than him knowing where the garbage chute was.