One excuse for the myriad of bad lions in art is that many of the (classical) painters had never seen a lion before. However, I think there's another reason so many lions are crap: the artists take advantage of the fact that no matter how bad a lion is, it is still recognizable.
Anyone who's played charades or draw something knows this. You can draw a dog and have people guessing "cow!" "goat!" "alligator!" endlessly, but lions have a handy mane that makes them instantly recognizable, no matter how many of the other features are completely fucked.
Case in point: I have witnessed a two year old easily recognize this as a lion:
Illustration from The Big Orange Splot, by Daniel Manus Pinkwater.
More importantly, first she recognized THIS as a lion:
So I guess what I'm saying is that lion artists are mostly both bad AND lazy.