whatson - 12:26 am on Aug 11, 2011 (gmt 0)
There may be many reasons, some obvious ones are time, money, knowledge. It may require a huge redesign, which can be expensive, along with a lot of reading and research. If the site is already performing well enough financially, then they may not bother. However, if Google puts their foot down (or appears to give that impression) that these changes must be done, then it will certainly make webmasters react more to it.