It was filmed with a guy playing Jabba with the intention of overlaying Jabba on top of the guy later- Jabba being a more Chewbacca-esque character at the time.
But they couldn't get it to work- technology was not up to the job. GL did not want Jabba to be human, so it was cut. Jabba, as we know, finally made into ROTJ but had been re-designed.
When he made the Special Ediiton of ANH- encouraged at first by a project to make Mos Eisley look better- it soon became apparent that this scene could finally be finished, and as an extra touch they added Boba.