Sorry that's what I meant, but why a different cast and paint job, why not just paint it to look like esb's?
(most of this was typed before Rafals answer, but Im leaving it in nonetheless)
I am by far not the best guy to answer that, but beeing relatively new here myself, I know how confusing this can be. So I'll try to at least point you in the right direction.
RafalFett or
boba87fett or
KaanE please correct me if anything I say is wrong.
Ok. Now regarding your question.
There were at least 5, more likely 6 versions of the Boba Fett helmet. The first one was painted by Joe Johnston (I dont know if he paintet the rest of the costume also?) that had ornamentation on the dome. This was sort of a prototype. Johnston ended up painting 2 more helmets, one of which is the helmet that ended up being used in ESB and later in the Special Edition of RotJ. 3 more were painted by a girl named Sandy, one of which is the helmet used in the original RotJ.
In my estimation, it must have been either an artistic decision to go with what is now known as the ESB suit and helmet for the production of ESB. Availability might have played a role, too, but with so many different suits available theoretically, that had as a whole a quite different feel, I am leaning towards artistic decision. The ESB suit stands out from all of the Boba variants as the most subdued version.
Then, between ESB and RotJ, likely damage happened to the suit. Definately the helmet, as that damage is now famously known as the RotJ SE helmet, and maybe also to other parts like the Jetpack.
That damage might have played a role in not using that for the filming of RotJ.
An interesting fact to note is, that they recasted the ESB helmet for the stunt helmets in RotJ and applied the "same" painttjob to those. So maybe the decision not to with the ESB helmet for the Hero RotJ suit came afterwards? (Although the ESB helemt at the point of the casting has already been damaged.)
Also, keep an eye on this thread:
*** Update 8/22/20 - This book has now moved from "The History of the Boba Fett Helmets" to "The History of the Boba Fett Costumes" *** Hi everyone, As some of you already know, I have been working on a photo book detailing the known history of the original Boba Fett helmets. I am hoping to...
www.thedentedhelmet.com
Hugely interesting and going to more or less contain the entire (known) story when its done.
Kind regards from Germany,
Ram