Because W3TC is insanely complex. We opted to use the Varnish HTTP Purge plugin and the Memcached Redux because it's simple, fast, effective, and there's no configuration needed (99% of the time - and we take care of cloudflare for you). In part, my feeling was that if someone WANTS to use it, but most people just want something that works out of the box. W3TC is awesome, but it's a lot of hands on tweaking and testing per site to make it work best.
We provide Pagespeed, which can do that. There is some latency with all minification, but in general if you're using Varnish (on DreamPress) then you really DON'T need to worry about minifying, since the load is cached on the Varnish server instead of Apache.
If you do want to use Pagespeed on DreamPress, read this: http://wiki.dreamhost.com/DreamPress#Can_I_use_PageSpeed_and_Varnish_together.3F
Nope! But.... The reason we don't use W3TC is also that it's different for everyone, so it's optimizations are going to be either unneeded (you don't need static file cache because of Varnish, and Memcached Redux does what W3TC does for Memcached anyway), or totally unique to your situation. Since that's impossible to automate, we opted not to.
Figure out WHY it's slow. I like to check a site real fast at http://www.isvarnishworking.com/ to see if Varnish is able to cache. if it's NOT then I know why my site's so darn slow!
Also we've been seeing some insane improvements on PHP 5.4, and we're totally happy to help you by bumping your PHP memory on DreamPress (which I actually just did for you)
Another thing to try is optimizing your database. WP is usually very good about it, but the same can't be said of all plugins and themes.