It still feels pretty weird to me that Ultimate Spider-Man declined so much, from being pretty often praised as the best version of character since the Stan Lee run (sure that amount of hype-praise sure couldn't last) to most people being indifferent to him including that he was ended/killed off.
The initial liking/praise I think was combination of liking that you didn't have 30 years of continuity/development for both readers and writers to have to keep in mind and that Spider-Man was again a high school teenager which some people felt is best version but over time after some 40, 60, 80 issues he developed quite a bit of continuity of his own and also people came to think that him being (always) stuck in high school was not such great setting/version. Otherwise there was just thinking that Ultimate Peter was so much like classic Peter that, especially with him not moving beyond high school, not much was lost with killing him off and with other characters, with Ultimate Marvel in general, fan reaction was often either Too Much Like the Original or Too Unlike, Too Just In Name Only the Original so either was pretty unnecessary.
The initial liking/praise I think was combination of liking that you didn't have 30 years of continuity/development for both readers and writers to have to keep in mind and that Spider-Man was again a high school teenager which some people felt is best version but over time after some 40, 60, 80 issues he developed quite a bit of continuity of his own and also people came to think that him being (always) stuck in high school was not such great setting/version. Otherwise there was just thinking that Ultimate Peter was so much like classic Peter that, especially with him not moving beyond high school, not much was lost with killing him off and with other characters, with Ultimate Marvel in general, fan reaction was often either Too Much Like the Original or Too Unlike, Too Just In Name Only the Original so either was pretty unnecessary.