I started to wonder why and here are some thoughts:
1. Processor speeds ventured into gigahertz territory, and maybe for some unknown reason, gigahertz doesn't sound so cool as it's mega brethren.
2. "Apple" finally succeeded in convincing general public, that processor speeds aren't that important. That was time when Macs were still running PowerPC processors, which were slower than their "Intel" counterparts. Irony is, that for some time now "Apple" is using "Intel" processors as well.
3. Or, most likely, computers have very much infiltrated the life of mainstream public. Fancy terms like "megahertz" doesn't mean anything to them and are difficult to remember. There are more important things like design, brand or color.
I suspect this should happen with any advanced technology. First it's marketed through technological advancement, but any tech advance "commodizes" over time and in the end it's always down to what color the stuff is.