G0LD3N 4RM That mod is amazing... but there are lots of factors that dictate which servers are full and which are not. A lot of is it location... people not pinging well, other servers being closer etc... heck, some people might prefer a vanilla experience or hell it might be an off day.
There is no doubt that the Reimagined Mod is great, but there are lots of reasons why servers running that mod might not be full all the time.