Because wordpress comments are being heavily spammed. You can use some plugin to make them DoFollow on your blog. I don't recommend doing so though
There are lots of plugin available on the web, which changes nofollow links to dofollow. check out this plugin: - http://www.semiologic.com/software/wp-tweaks/dofollow/ Thanks...
To offer do follow you will naturally have to be hosting the blog yourself. But as a commentor theres not much can be done about it. do follow, dont follow, is only to do with PR of course.
Because you blog will be spammed just for PR juice and your articles will loose there serps with the loss of PR.