Think a lot comes down to local interest/competitors (in both rugby and other sports) and facilities. My 2 pennies worth based on travelling to most of the grounds over the last few years, so just my opinions!
Coventry - have gone from strength to strength since their promotion, and imo have a great chance of competing with Ealing for the title next year. Great facilities, really easy access to the station etc. And how many casual Coventry based Wasps fans have drifted their way in the last 2 years?
Bedford - consistently finishing top half and normally in the top 2-3 over many years - play great rugby. No local Bedford football club outside of the football leagues so nothing to really compete with.
Doncaster - generally top half finisher, amazing facilities (the only club to apply and pass the RFU audit every single year. Sadly they dont get near promotion as their facilities are top notch).
To compare to some of the strugglers:
Ealing - competing with the over saturated London sports market, Sarries etc. Regularly finish top but no hint of promotion, smash most of their opposition in lop sided uncompetitive games - wheres the fun in that?
Hartpury - glorified University team and Gloucesters feeder club. A real real ballache to get to on public transport.
Ampthill - lovely local club but not yet established FT in the championship. Ground is effectively the side of a hill in the middle of the woods with a clubhouse.
Caldy - pretty much the same as Ampthill in terms of being established and facilities.
London Scottish - Have struggled massively over the past few years, crowds dropped as the results went. Competing for fans with Richmond, Harlequins etc, as well as the same London sports market. Share the RAG, which is falling apart.
Its an odd league with some big skills gaps between the top and bottom (most of the clubs with big attendances are fully pro), vs the semi pro/amateur clubs in the bottom half, as well as big big differences in terms of stadiums/facilities/match day experience etc.