I would like to chime in and say that I have observed similarly to what whooami posted about the userbase of WP vs the userbase of phpBB.
phpBB is a bulletin board/forum system and its whole purpose is to cater to groups and communities. It's amazing what people can do with phpBB and other BB systems from setting up role-playing communities to support forums to entire websites.
WP, on the other hand, is a more single user-centric application and it seems that the greater majority of users utilize WP to set up personal or business blog sites and single-user websites.
When you compare phpBB to WP in this way, you can see how phpBB's support system might be stronger if only because of the nature of phpBB and the types of people and types of projects it attracts.
Community vs single-user.
Share vs keep to oneself.
In my opinion, the more personalized single-user nature of WP invokes possessiveness over code and design which can translate over to less willingness to offer support from some users because there is fear of other users copying what they do.
This is an issue I have noticed specifically in the design and front-end department of web development (and WP falls into this category on a lot of levels, especially when talking about theming), but phpBB and other similar systems seem to escape this mentality more due to the community nature of the application itself.
Also, the solitary non-community nature of the WP application in itself means that a lot of people come in, get help, maybe offer some help, and when their WP project is done with, leave.
I've used WP for a few years now and I admit to being guilty of coming and going. Not because I don't want to stay and help, but simply because once a WP project is finished, it is usually finished for good and my attention is being focused elsewhere due to a change in project.
With all that said, I feel that it is a bit unfair to compare phpBB to WP in terms of support.
Something I DO feel that contributes to a potential issue with WP support (lots of people posting the same support questions/issues and sometimes getting slower answers because it's an old repeated question, for example), however, is that I think a lot of users install WP with hopes to theme on their own... only to find out LATER that in order to successfully theme WP, it is almost a requirement that they have an extensive knowledge of XHTML and CSS and can at least read php.
I have seen before that a lot of users show up bright-eyed and bushy-tailed - all eager to learn how to theme WP - and end up leaving looking like they were hunted by bloodhounds because they realize that without extensive knowledge, they can't make WP do what they want it to do or look how they want it to look.
It's a shame for that to happen, I think, because WP is a wonderfully flexible application/platform to work off of and presents to the user a canvas that allows the user full control of both content publishing and content presentation while being completely usable with the majority of features intact upon first install.
Personally speaking, I have seen some great support in the forums for WP, but if there was one thing I had to address, it would be the issue I described above.