I think it's because the 'search' button is hard to find!
It's very small, and up near the top, near the advertisements.
I've always thought that having four distinct rows of 'buttons' (i.e. links) is unintuitive. Particularly the top 3.
At the top are the buttons that say: home, help, search, profile, calendar, logout
Then there's the block of text with the forums
Then theres a row lof links for: Rules, Old Boards etc
Then there are the buttons to: mark as read, new topic etc
But, whatever. There are some forums where I can NEVER find the 'reply' or 'new topic' buttons/links, so clearly there's no perfect solution. But I think moving the "search' from the very top (where it sits with a bunch of buttons I NEVER click) down to the bottom (and most obvious) row of "mark as read", "new topic", etc might help.
But who knows. If people want to search, they'll figure out where the search button is.