Two reasons I haven't bought an SSD for my machine:
1. Price
2. The technology was changing very rapidly for a while, which looks to finally be slowing down indicating some level of maturity.
That being said, both of my reasons are rapidly disappearing and I think I will be making an addition to my primary rig in the coming months. I think people who are looking at SSDs see their benefits, the cost vs benefit ratio isn't always enough to sway someone over to buying an SSD. We will see, but as prices creep down you should see wider adoption of SSDs.