I can see why some would think that way, and I can probably accept that, too. But to offer a bit of a counter-argument - not offering a motivation for Joker can also be a cop-out - Joker does whatever the writer wants him to do, he doesn't need to bother with any consistent motivation - that can lead to lazy writing.
There is a difference between saying, "he has a motivation," and, "he has a specific mental illness".
I will note, though, that a major theme of the Joker is his unpredictability. He is scary in large part because you don't know what he will do. Give him a strong, recognizable motivation, and he becomes predictable, and thus less fearsome.