Also people tend to forget that Luke's black outfit in ROTJ was supposed to represent his brush with the dark side. Of course him actually tuning evil was eventually written out, but Luke did have his dark side moment while fighting Vader in the end.
As far as the lore is concerned I can't remember having ever read that it was supposed to be an actual Jedi outfit. I'm pretty sure it was just something he put together before meeting Jabba (even the pre-PT EU added a practical explanation for the outfit.)
And of course when we see Anakin (Sebastian Shaw) at the end of ROTJ he's wearing the same moisture farmer outfit that Obi-Wan wore in ANH. Not to mention that Yoda more or less wears the same outfit. I don't actually now if this is a pre- or post-PT thing, but the reasoning behind the Jedi both in the OT and PT wearing moisture farmer outfits, or rather just a casual outfit common among colonists and farmers, were because they were supposed to be ascetics; monks more or less. Hence the humble outfits.
And Lucas was quite adamant back in the pre-PT days that the Jedi order was basically just warrior Buddhist monks, even comparing them to Yogis once.