Ars Technica has a great article giving the reasons why HTTPS is not used more on the Web.
The summary is as follows:
1) HTTPS adds some latency to initial connection establishment.
2) HTTPS documents cannot be cached by the browser. While this is not a big issue when the client and server are on the same continents,
3) HTTPS websites cannot be virtual hosts on the same server, and this causes addition in operation costs.
It goes on to add that while these are practical reasons why HTTPS isn't widely deployed, the practical hurdles will fall away eventually leading to a wider adoption of HTTPS.
