I’m pretty sure the LPP used for the SSE is a duplicate, so it’s 4(?) generations away from the negative.

Even if it isn’t a dupe, it’s still 3 generations away so it really shouldn’t come as a surprise that Bluray based sources have more detail and less grain than a scan of a release print. But like santakrooz says, release prints are what people saw in the cinema and Cinema prints were not struck from the original negative - contact printing was a destructive process, and if they created hundreds of prints from the negative it would have been destroyed. So they would create an interpositive, and use that to create a throw away negative to use for striking prints. When it wears out, they would simply create another. This allowed them to create hundreds of prints without damaging the negative. Sometimes a few prints would be struck directly from the negative for critics screenings, and special events, but these did not end up at your local cinema.

SSE = Original negative -> Interpositive -> internegative -> Positive print -> duplicate -> HD print scanner.
4K77 = Original negative -> color separation matrices -> Positive print -> 4K print scanner
Bluray = Original negative -> HD print scanner.

(According to Videography, the negatives were scanned on a Cintel C-Reality telecine, at 1920x1080 resolution, in 4:4:4 RGB, recorded to Sony SR tape in 2004)
Source: https://web.archive.org/web/20150219050324/http://secrethistoryofstarwars.com:80/savingstarwars.html

It would be really nice if Disney/Lucasfilm could dig up one of those original IPs and create a new 4K scan - it would be head and shoulders above anything else we have access to.