NME caters for young music fans; they have to keep looking interesting by exposing new bands that they "discovered" before every other music paper, & claim each of these new bands they put on the cover is "the future/saviour of rock'n'roll" or "what you've been waiting for".

There needs to be a quick turnover to maintain their young & hip image, so they need to show new flesh as often as possible because they wouldn't look cool if they admitted most new bands are crap & don't deserve to be on the cover nor even a full-page live report. Giving too much importance to already established bands would imply there's not much novelty worthwile, so no need to read a music paper every week.

There are a few exceptions for veterans they still display prominently on the front page, like Oasis, because they know any issue with Liam or Noel on the cover will grab many fans' attention, but their main line of work is clearly new/recent bands. Serious music fans read Mojo or possibly Q Magazine, not the NME which has to discover a new star every other week.

Hence NME's top 50 albums of the year, which must be in line with their editorial line, otherwise they'd admit they scammed their readers all year long by proclaiming the 1975 are the next big thing or giving so much coverage on the Courteneers recording a new album.