"Ok, I see this get asked all the time, and I'm going to answer now in an effort that this can be used as the answer moving forward.
First, you have to consider the context and timeline of Star Wars products as they were released. When Star Wars came out, Darth Vader was the baddest villain people had ever seen. He had strange, mysterious powers that no one new the limits of, no one knew an "emperor" even existed yet, and Vader he has no remorse and is willing to off anyone on either side that gets in his way. There were two years before Empire came out for people to see Star Wars 100 times and continue to build up his legacy.
Then Empire came out. We see Vader choking out his officer's, toying with, then beating the crap out of Luke in a fight, and his Empire bros pretty much stomping out anything the Rebel's threw at them. We do learn that he has a master (very intriguing, however who knows what's going on here yet). Basically he just reconfirms and builds on his badassery throughout the film. Now, halfway through the film, we see Vader is stumped and needs a bit of help. He calls in a group of people we've never seen before "bounty hunters" and is basically directing all of his attention at one dude. This dude, is quiet, calculating, the first person we've seen to not be nervous around Vader, and in fact, Vader seems worried that he may be too dangerous hence, "No disintegration" that line alone just begs for a crazy background story to be told. Also, he responds with a passive-aggressive "as you wish", as though he could barely give a care about what Vader thinks. Also, he has cooler armor than anyone else, all sorts of weapons and gadgets, and seems like the first non-force owner that can hold his own with the Dark Lord. Not only this, but he's the smartest person in the film. Han (one of the current craftiest people we've met) outsmarts Vader and the empire, but Fett isn't fooled. Fett deftly plays both sides and gets his man. Finally, in cloud city, he twice talks back to Vader with no regard, and Vader just takes it. The respect he has for this man compared to everyone else in the universe is clearly next level.
So fans have two more years of seeing this, mulling over it, and debating who this dude is. Return comes out, and we finally see the flamethrower, his lack of concern with going toe-to-toe with a Jedi Knight, and then unfortunately, the jetpack in action. This part sucked and everyone was pretty pissed they ending him with a fart joke here.
So that was 1983. The prequels did not start coming out until 1999, with the Fett story untouched in movie form until 2002. This means there was about 30 years of Star Wars love, with people analyzing and micro-analyzing the films and characters. With the popularity of the stories and the toys, Fett emerged as a fan favorite despite his "death". It was a cool toy, with cool armor, a cool ship, a mysterious backstory, and the only person to put Vader on edge. Sure enough novelizations came out trying to fill in the gaps and point to how he earned this respect, what happened after the sarlaac pit (spoiler alert- he lived), and how he continued to run train on anyone that gets in his way. These stories were considered cannon and an official part of the Star Wars universe. It held up on our beliefs that Boba Fett was the most badass non-Jedi in the Star Wars galaxy.
So for people that grew up with Star Wars, and especially those who got into the EU, Bobba Fett was next level. Then, unfortunately, the prequels came out and changed the lore. As a ploy to play on the Fett fandom (and sell a new color scheme of Fett toys), Lucas included Fett randomly as something different from the EU stories, made his dad the clone DNA, and basically reduced his character (and his new father's character) into bumbling shadows of what we had expected after the last 30 years. Additionally, the prequels worked to reduce the baddasdom of Vader, rendering the impact of standing up to him (like Boba did in the OT) less of a big deal if someone has seen the prequels and has this new view of Vader. One example- in A New Hope, what we are first told about Vader's past is he hunted down and destroyed the Jedi Knights, leaving them all but extinct, and ending their 1,000 generations of protection. What a BA! However, in the prequels, all he does is to the Jedi's is take a cheap shot at an unexpected Mace, kill some younglings, and lose in a fight to Obi Wan. Where is this badass we heard all about? The clone troopers did all of the heavy lifting, and if you ask me, Anakin was meaningless in the Emperor's rise to power. What did he add? So now with a less cool Vader, and a less cool new backstory, things started to change for poor Boba.
I can understand how someone who grew up in the prequel era does not understand the Fett love, and to be honest, I wish he wasn't included in the prequel's at all. It's a bummer what they did to him, but hopefully the new Star Wars can return some of his previous glory." -/u/Drew312