For whatever reason, people found him interesting. And yes, he does actually play a pretty large role in the series, since he tracks the Millennium Falcon and delivers Han Solo to Jabba the Hutt. I think it mainly had something to do with his armor and appearance. Either way, they went on to flesh out his character in comics and books which made him a rather deep character.
One of the first things I noticed after entering Star Wars fandom world was fan's obsession with Boba Fett.
The intrigue of this character was spawned from his appearance, lack of lines, and perhaps the fact that he had a Jet-pack...
I never understood why so many Star Wars fans gravitated towards him but his character was drawn out even further between the prequels, novels, video games, and in general the expanded universe.
He never did much of merit in the original Star Wars movies. Yes he captured Han Solo and delivered him to Jabba but he was such a minor or low profile character.
Now though his character has vastly expanded beyond the movies and he is a near superstar.
I think Darth Maul could have been idolized the same way if his character did not die so quickly. But the main reason I think for him not being a "glorified minor character" in the prequels were simply because Episode I simply could not match the nearly immeasurable hype.
As long as I've been in the SW fandom, I've yet to fully understand the obsession with Boba Fett. I've seen an interview where George Lucas stated he was amazed by the amount of attention that Mr. Fett has received.
I believe it has to do with the lack of major dialogue, his costume design, and the over all mysteriousness of the character (up to Jedi's original release).
Although, from what I believe, Boba Fett actually made an appearance in the 1978 SW Holiday Special (I never watched it all the way through so I can't confirm that sighting).