Good question. I personally didn’t care for Puberty 2. - thought the singles were great but as a whole it didn’t break much new ground from Bury Me at Makeout Creek and was only seldom as hooky. I think publications like Pitchfork had previously seen her as promising and gave her moderate press (I first discovered her through this op-ed from them) but waited to really pull the trigger on spotlighting her (i.e. Over/Under appearance, interview, news stories when she expands tour dates or shares new work etc) until Bury Me had accumulated its cult following and “Your Best American Girl” blew up, hence the 8.5 for Puberty 2. Maybe other publications started to wave-ride for Be the Cowboy, but to be honest the critical evaluations of the two records look pretty similar - this one just seems to be accumulating a few more year-end listings which I think is at least partially because of 2018 being kind of a dull year for music in comparison to 2016.
Just my opinion, but maybe what also makes Be the Cowboy such a golden opportunity to symbolically recognize her overall body of work is that it’s a solid marriage of the artsy aesthetic from her first two records (which she made while studying music at Purchase) and the indie-rock of her post-collegiate releases. I think with Bury Me at Makeout Creek you could sort of tell she was still learning how to play guitar because so many of the songs use simple open tunings and verse-chorus structures, but the new record is allowing even more of the weirder melodic turns (i.e. “Why Didn’t You Stop Me?”) and time signatures (i.e. “Old Friend”) to seep back in. You also get songs like “A Horse Named Cold Air” which, for all intents and purposes, sounds like it’s from Lush, her first album. If there was ever a Mitski record once could simultaneously view as a summary of her output up to this point, Be the Cowboy would be it.
I think I mentioned this in my review but the two biggest critiques I’ve seen of the record are (1) it’s too stylistically inconsistent and (2) most of the songs are too short. However, for some critics I can see those actually being stylistic strengths that help differentiate the album from being “just another indie rock record” and give publications a solid reason to slot it at number one. I think a lot of the mythologizing of cowboy imagery (slightly reinforced on the album with tracks like “Lonesome Love”) that Mitski’s done in interviews, her Trevor Noah appearance etc. also helps the record come across as more conceptual than it actually is - another thing critics totally eat up. I 100% agree that her high ranking on the aggregate list is at least partially influenced by the massive clout she’s gradually built over the last four years, but I do think in a vortex the album alone has a few traits that make it real critical fodder.
Overall I think the new record is good, and “A Pearl” might be her best song of all time so I’m glad it’s receiving praise.