Advertising is the big impediment for anyone that is ad-driven. Most ad networks don't provide HTTPS for ads. Kind of infuriating in this day and age, really.
There are technical considerations - until we have full SNI coverage (Android 2.x is the big hole there right now), you can't really run HTTPS on shared hosting (or any place where you have multiple domains on the same IP).
And there are financial considerations - while an SSL certificate isn't expensive, if you're using a CDN or doing any kind of otherwise serious data delivery, SSL is many times more expensive per-byte than HTTP is. If there isn't financial risk mitigation gained by switching to HTTPS-only, it may not make sense from a business perspective.
There are technical considerations - until we have full SNI coverage (Android 2.x is the big hole there right now), you can't really run HTTPS on shared hosting (or any place where you have multiple domains on the same IP).
And there are financial considerations - while an SSL certificate isn't expensive, if you're using a CDN or doing any kind of otherwise serious data delivery, SSL is many times more expensive per-byte than HTTP is. If there isn't financial risk mitigation gained by switching to HTTPS-only, it may not make sense from a business perspective.