Canon EF 24-70mm F2.8 versus F4.0

From what I had read in the past they perform similarly if you are comparing the 2.8 II version, and you should expect that to be the case, being that they are both L lenses. The biggest difference her is that you pay a decent bump in price and loose IS to get a 2.8 aperture if you want it.
There may be a few on he form who personally have used both lenses but I am also sure there are reviews online where people compared the nuances of them.

 

Its not really that big of a surprise, the wider the aperture the more complex the lens, you combine that with features like IS and USM etc.. and try to keep the lens reasonably compact, built with metal instead of plastic weather sealed, etc.. There are trade offs that will happen or they have a price tag to go along with the spec list... You get a pro grade zoom with IS and a 2.8 fixed aperture they cost above the $2000 price point.

I think it is as simple as the 24-70mk1 had CA and reliability issues over the years after its 2002 release, but was a favorite of wedding photographers, so they stepped up and produced a better performing mkII version in 2012. I do not think they were that concerned with adding IS as they did not need it for a premium price tag and it was not likely the biggest need of a short zoom lens being shot wide open at 2.8. Canon did follow up right behind it with the 4.0 IS a few years later. I'm not sure how well it would sell adding another option of a 2.8 IS would do after the fact with its own price point that would drive down or compress the prices of the others. Similarly they also have introduced 2.8 or IS versions of the 16-35 zoom but not a 2.8 IS. This is all about ROI and market need with Canon I'm sure..