Hey, it's me. said:

It's a ridiculous point anyway and I WAS being mildly sarcastic C3PS. I mean at that juncture how was we (the audience) supposed to know that what Obi-Wan was wearing was standard Jedi attire?

I assumed you were being completely sarcastic.

Just one of the many flaws of the prequels. Throughout the OT, we see three characters wearing what we now call Jedi robes, Owen, Obi-Wan, and Anakin's ghost. And maybe some other characters on Mos Eisley? When I was younger, I always got the impression robes are just what people wear in the SW universe, especially middle aged to older men, it seems robes popped up a lot on regular characters in the old SW comic books as well.

Like Doubleofive said, it seemed it became the Jedi outfit in the prequels simply because it was what old Obi-Wan, the only Jedi we knew wore, disregarding the fact that many other people on the planet he was hiding on seemed to dress the same way.

The fact that Sebastian Shaw's Anakin is wearing similar clothes could indicate that they had already decided that this was the standard Jedi attire as early as the time ROTJ was made, or, as makes more sense with Owen wearing it, that it was just standard galactic attire.