I came across Drupal just as 5 was in beta. I immediately fell in love, as many of you have. I was, at the time, in a position to do Drupal work full time. At the beginning of the year, that changed and so, with a job change, I began to fall behind in my module issue queues. One feature requested on all of them, of course, was to upgrade to D6. The problem was that I didn't have time. I still did, and still do, Drupal work on the side and have been paid to update a couple of the modules, but non-paying work hasn't been getting done. Other of my mdoules have seen updates thanks to new co-maintainers.
The major obstacle in upgrading the modules are 1) the sheer number of changes required, 2) the complexity of some of the changes, and 3) the dependence on modules that to date, still haven't seen final D6 releases. Primarily, point three consists of the big 3 contribs, CCK, Views, and OG. I mean, only one of those three is 'done' for D6 and D7 is right around the corner. How can anyone be expected to maintain modules when the core (and core contrib) code is always in such flux? Add to that no backwards compatibility, and any given module only has a one year life span. That seems pretty crazy and very much not enterprise-grade.