Major difference: LotR is almost perfectly written and has no asspull cop-outs.
It may not be GoT in terms of death frequency, but Tolkin also never wrote fake death scenes for his characters (Gandalf's "death" and "resurrection" is perfectly explained in terms of lore) and let the few main characters he wrote death scenes for actually die (Boromir, Theoden, Denethor, Saruman, Grima, Gollum).
Not to mention that the Silmarillion (prequel) is actually way darker and more bittersweet than Oda could ever write.
Oda pulls out his bs fake tension and fake death scenes over and over again.
Plain and simple: Bad writing.