A couple reasons I assume for this
- What a theme can do is really limited to html, css, js. You cannot extend / provide any functionality or CMS integration. So if you have a 3 column theme you'll have to hard code the content in the template. No way of adding those extra components to the cms. So even through the user can download a theme they have to then write PHP (or copy it). Not exactly user friendly.
- SilverStripe attracts a much more developer audience rather than say Wordpress. The learning curve is much more techy and because designers love easy, usable software perhaps they get turned off by things like customizing modules via PHP, writing cms extensions rather than just having to download.
- SilverStripe attracts a much more enterprise level audience. Most of the developers who work with SilverStripe that I talk to implement it for their clients but for their own blogs etc they prefer wordpress or they aren't the type to work on themes on the weekends.
It would be good to see some new themes online. If you have any other comments on top of those would love to hear.