Why not to use one suffix for both stems? Like 'nervAL'/'mentAL' or 'nervOUS'/'mentOUS'. Thanks for your answer
It is impossible to trace the logic behind the adoption of usage. The best we can do is look at the etymology per the OED.
There are actually two words in English spelled nerval. The first (now obsolete) came into the language in 1400 via medieval Latin (nervale, the neuter of nervalis) and meant an ointment for the sinews. (Remember that this was before modern neurology.) The second is documented from 1636 and means affecting the nerves, what we would call neural. This word may have the same derivation or it might have come directly from the French.
English words can have a double path from Latin, the direct route and the detour through French, and we have to look at both languages to check the derivations.
Nervous came into English via the Latin nervosus, first in the sense of sinewy (like the obsolete nerval and at about the same time) and took until the 1700s to develop the sense of the second nerval, i.e., affecting the nerves.
Why did one form supplant the other? Alas, there's simply no answer to that.
Mental comes to us ultimately from Latin mens, mentis (the mind) via medieval Latin's addition of the Latin suffix -al meaning pertaining to. There was no Latin -ous form to give us mentous
Why don't we adopt mentous in imitation of nervous (or drop nervous for nerval)? Alas, there's simply no way to do that.
Because natural languages are what they are: man-made, far from perfect, sometimes clumsy, but nevertheless invaluable to communicate between humans. Est usus qui facit linguas, languages are made by the people who use them - and those people need not to be literate or exceedingly logical, they just need to talk and to get understood.
Nothing forbids to build up from scratch a new, optimised, entirely logical language - actually it has been done several times. But unless a real lot of people start using (and, of course, abusing) it, it will stay as tasteless as distilled water.