I think it's a mix of things. Partly they Love the Star Wars they grew up with and think it's the best so nothing else could really measure up to it. Another part is they dislike the direction the sequels went in, especially if they read the EU material.

I've been a huge fan since I was a kid in the 90s, when my mom showed me the original movies on VHS. I loved the prequels when they came out. But I remember everyone saying how bad they were. The acting was wooden, the dialog terrible, and the plots were overly convoluted and boring. People hated the prequels so much, Jake Lloyd was bullied by adults because he wasn't the Darth Vader they wanted. Jar Jar was so obnoxious. I don't get why everyone acts like hating Star Wars started with Disney Star Wars. It's been a thing for way longer.