"Come on, El Reg, it's not that hard to implement!"
It's an anti-spam measure. For the same reason commentards can't post HTML at all initially, and then only a small subset. The more posts you make the more HTML you get to use. And as an IT/technical website, commentards are expected to be able to cope with a little manual HTML if they think their comments deserve it.
"You can add hyperlinks in your post - once you have had 100 posts accepted for publication. We have set the bar high to deter spammers and enable us to assess use. We will review this bar from time to time."