I disagree that all films post-Aliens are bad (Alien 3 and Covenant are good, Resurrection is okay, Prometheus is bad, IMO), or that the sequel trilogy is bad (highly flawed, but not bad - the only genuinely bad SW film I've seen is The Clone Wars), but semantics aside, I'd say the differences are:
1) Star Wars is a much larger IP, so you're going to get more people saying one thing or another. So when people lost their shit over Last Jedi, that's a lot of people.
2) There's the continuity issue. The sequel trilogy retconned the old EU from existence, and that peaved some people, not to mention that the sequel trilogy didn't do much interesting with its setting. In contrast, the Alien EU is a mess. Prometheus retconned the mala'lak into the Engineers, down to their very biology, but few people brought that up, because the EU was already highly inconsistent.
3) A lot of people will disagree with me, but I'll maintain that Alien (and by extension, the wider "Xenopedia" IP) is a setting-driven IP, while Star Wars is a plot-driven IP. And, by extension, character. So with the sequel trilogy, you had people enraged that Han died, or that Luke's character was "ruined." In contrast, Prometheus and Covenant don't 'ruin' Ellen Ripley or anything like that. You can argue that it ruins the mystery of the xenomorph (you could also argue that the EU already did that, but see point 2), but the result is that the sequel trilogy had baggage and obligations that the Alien prequel films didn't.
4) Expectations differed. My own personal thoughts aside, there's consensus that the Alien films peaked with Aliens (even if Alien is your preferred film, one can't deny the positive reception to Aliens), and since then, it's been films that, whatever some may say, have never lived up to the originals, so even if Prometheus and Covenant were the worst things in the world, that wouldn't be breaking trends, so to speak. Star Wars though, I'd argue, doesn't have consensus in the same way. Everyone agrees that the OT is the best thing in the history of ever, but aside from that? The prequels were generally well received when they came out, then they were revilved, now people seem to like them again after the ST. It's why I suspect that the sequels will have a re-evaluation later down the line, especially as kids who grew up with them get older.
There's also the fact that everything in the Alien IP uses Alien or Aliens as a template, and very little differs from it. Star Wars though...Star Wars has been everything from space fantasy, to military sci-fi, to even space horror. People love Star Wars, but while it's a larger fanbase, I'd argue it's a more fragmented one. So the result is that no matter what Disney did, it would piss people off.