1. While you (and a few others) may be interested in it, a large percentage (perhaps most) of the market won't be. So they don't consider it worth the effort to build out just to support a tiny percentage of their overall userbase.
All that engineering time/effort spent on a paid subscription system could go towards features which more people would use.
2. In some cases, the value of the user to advertisers is higher than they'd be willing to pay for an ad free experience. Probably not much of the time, but I remember hearing that a paid Facebook account would likely be something like $70 a month/year/whatever to make up for the loss in ad revenue.