It's probably worth noting that Haynes no longer list the book in their catalogue, so ultimately they wouldn't receive any additional sales etc. through more advertising. I wouldn't be surprised if the discussion on pulling the forum came at the same time as the decision to remove the book from the catalogue.
I can also understand their reluctance to pass the forum over to a new owner outside of their organisation, as their actions could have a direct impact on their brand if the new owner went in a direction that they didn't agree with.
The kit car industry in the traditional sense seems to be declining. When you look to see what's available to the general punter who has £5k+ burning a hole in their pocket for a 2 seater sports car, there's a lot of choice out there. I've spent more on my Roadster (that includes stuff, tools etc. that I've bought directly because of it) than buying my mark 3 MX-5, which will do 80% of what a Roadster does but it has lots of other positives that the Roadster isn't so good at. It's something that you can drive in bad weather comfortably, you can take to the shops and not worry about people climbing in to take photos in etc. etc.
Heck, even for the cost of taking a car for IVA and registering it, you can get a reasonable mark 2 MX-5...