From people I know reasons I've heard: 1) They don't know that registration is possible or don't know how simple it is 2) They worry that if they register they will end up taking too much time editing Wikipedia. You could ask a few editors on fixed IP addresses. There are enough that you might get some useful data.
Quoting Phoenix wiki phoenix.wiki@gmail.com:
Intersting. I wonder why regular ips don't register. I edit occasionally as an ip for a while but created an account once my edits became more frequent
Phoenix 15 _______________________________________________ WikiEN-l mailing list WikiEN-l@lists.wikimedia.org To unsubscribe from this mailing list, visit: http://lists.wikimedia.org/mailman/listinfo/wikien-l