Apart from people moving on from E90 and not remaining here, the earlier years had a different style of conversation, partly due to the fact that all the cars had BMW warranty so people weren't joining to find how to get cheap parts or worry about strange noises/vibrations and the like.
Since it was much newer then, lots of threads on various subjects were started that had never been previously discussed so most conversation was reasonably fresh but now we get lots of posts from newish members asking questions that have been asked and answered dozens of times already.
is the obvious response to people who ask old questions but I have some sympathy because I don't think the search function is very good when looking for something too specific.
The off-topic forum is probably the freshest of the four forums since it doesn't suffer from the above problems.
...You also never hear from the UK forum moderators any more - which is a pity.