*loads philosophical modus*
Okay.... this is how I see it:
(Don't worry, I won't tell ya anything like "in the 70's the models were.... bla... no cgi.... bla bla....")
Everything that is non-empirial made really a step back... Rebels and such guys (pirates, outlaws,...) has to use old vehicles, repaired vehicles,...) So there won't be any crazy looking tridroid-style fighters with them....
The Empire itself got its fleet, with a particular type of mass produced fighters (the standard TIEs) and carriers (the ISDs)...
We see that they are powerful enough (at least the empire thought this... ;)), so there is no need to have a bunch of various colorful childish looking fighters or vehicles.... ;)