Because those threads serve a purpose and have enough people interested in a specific category to warrant their own discussion thread. Take the Steamshots USA thread: that thread is a specific category that gets a decent number of posts every week, thus it remains open. No one wants what basically amounts to a “meme thread” on the forums. Calling a skin cursed is subjective IMO. Reskins go in the reskins thread.
Because they are specific topics that people use and sometimes help each other with, on modeling specific themes/eras. There is stuff that people put actual work into, stuff that took time to do. Plus it was a pointless thread that served no real purpose. The moderators have made up their minds.
Because they deal with specific Uk railway time periods and locomotive types. Pre-blue covers steam and pre-1960s diesels and electrics from the pre-grouping, grouping and BR eras. Steam covers all Uk steam engines and BR blue covers Uk railways after 1960. In general Uk Trainz folk stick closely to their chosen forum and don't stray into other Uk forums because they don't hold any interest for them.