Most handbooks start out as smaller works in progress, and the thread itself is then used to debate and highlight other tactics, feat choices, builds, etc. Doing it in wiki format doesn't offer the same convenience for "living documents".
Now, possibly you could work on them for a while before copying and pasting them over once tehy're done. But it's never truly "done", and copying and pasting isn't as likely to be a labour of love like writing a handbook.
There's also visibility - put a handbook on the CO boards and that's where people will see it, and where less regular visitors will look for it. When not even all the CO regulars know about the wiki (and while I know about it, I never check it) it's never going to get the same level of traffic.
If it takes off from the beginning of 4e, and if the various handbooks end up split between here and gleemax, then a central wiki for them might make more sense.
Tbh I doubt it, but to increase the chances make sure it's got the same functionality on offer as the wotc boards (sblocks, etc), and make it well known.