The argument that fans just want the same thing as before is a tired one that falls apart with even the slightest examination. Breath of the Wild, AC Black Flag, Devil May Cry 3, Metroid Prime, Resident Evil 4 & 7; all beloved sequels to pre-established series that introduced numerous drastic changes to the usual formula. Or, if we're just considering movies, Logan, Thor: Ragnarok, and the Dark Knight are all notable departures from their preceding films, and have been widely praised.
The primary reason why innovative movies, and in particular, The Last Jedi, are criticized so widely is because they add a number of new elements, but don't implement them very well, and end up as a worse movie as a result. TLJ introduces or expands upon a ton of plot points/characters, Luke's history with Kylo Ren, child slave trafficking, the relationship between Finn and Rose, tracking down a codebreaker to get through star destroyer shields, Rey's connection with Kylo Ren, Phasma, Snoke, Poe, Leia, that one Rebel commander lady whose name escapes me, and then on top of all that, tries to add some context to the current state of the universe and the history of the Jedi. There simply isn't enough room for all of this, even in a movie as long as TLJ, and the end result is a bunch of plot points and character arcs all competing for screen time, and none of them getting the development they need. The fact that the movie spends a solid 5th of its run time on a trip to a space casino that ultimately barely contributes to the plot in any way simply compounds this. TLJ needed to pick maybe five of these, spend its run time really fleshing them out, and then have a well executed setup to a final installment with some well developed characters who people are invested in. As it stands, we head into the final installment with two main protagonists who barely have any personality or motivation established, a villain who has no motivation beyond teenage angst, and a set of side characters who have even less development.
After TFA, one would think that there would be a number of priorities for its sequel. Fleshing out the characters of Snoke, Finn, and Rey. Give Phasma some screen time after she was so thoroughly ignored in TFA. TLJ gets around to, at best, two of these, and one hardly counts considering that they take the time to characterize Snoke and then proceed to immediately kill him off. What is the point of establishing an Emperor-esque figure that is so much more powerful than everyone if you're just going to off him not even halfway into the trilogy?
On top of this, there are plot holes on top of plot holes, probably as a result of the movie trying to juggle so many plot points. Why can Leia magically float through space now? How did Rey develop competent lightsaber wielding and force using skills within the span of a day of lessons? It took Anakin years to get there, so what makes Rey so special beyond simply having "raw power." Why does Luke not tell the resistance that he's going to buy time for them to escape, and instead has to rely on Poe being able to intuit his plan? Why does creating a projection of himself kill Luke? Where the hell did Snoke come from, and how did he get to be such a powerful force user? And still, how the hell did Luke/Anakin's old lightsaber wind up in Lemon Head's possession? Again, these could have been addressed if TLJ made better use of its time. But it didn't, and now Episode IX has a ton of baggage from the previous two movies along with whatever storyline it carries out, and chances are a significant portion of these will be dropped and never heard from again.
Wanting to do something new doesn't make TLJ a bad movie. Introducing new elements to the series doesn't make it a bad movie. What makes it a disappointment, at the very least, is the fact that it introduces so many different things that it's ultimately unable to give any of them the attention they deserve. What we're left with is a tangled mess of underdeveloped plot threads and half baked character arcs that ultimately leave more questions than they do answers.
