Here are the reasons:
- I now own a Canon EF 24-105mm f/4.0 L IS USM lens. I got this as the kit lens when I purchased my Canon 5D MarkII camera. Though it gives up one f stop, it has image stabilization, so I can shoot hand held down to 1/10 sec. in most situations. It also has an additional 35mm in focal length reach.
- I have a preference for prime lenses, and I own a 24mm f/1.8 Sigma, a 35mm f/2.0 Canon, a 50mm f/1.4 Canon, and an 85mm f/1.4 Sigma, as well as a 90mm f/2.8 macro by Tamron. These primes cover everything and more that I gave up in focal length, plus give me greater low light capability than the f/2.8 lens.
- The 24-70mm f/2.8 L zoom is a heavy, large lens that I got tired lugging around all day. It is 10 oz heavier than the 24-105mm f/4.0 L
- I also own Canon 40D that has 1.6x sensor. I often put the 24-105mm f/4.0 L on it and get a pretty respectable range of about 40mm to about 164mm in equivalent full-frame focal length. This gives me a much longer reach on the 40D than the 24-70mm zoom allows.
So, weight and reach were the primary reasons I kept the 24-105mm f/4.0 and sold the 24-70mm f/2.8. For low light photography, the image stabilization on the f/4.0 more than makes up for the loss of one f stop.
Both lenses use the same 77mm filter size.
How do the two lenses compare optically? Both lenses are as good as it gets for zooms, fully living up to the reputation Canon maintains for its line of L lenses.
You can find a comprehensive review of the EF 24-105 f/4.0 IS USM lens, and a more detailed comparison to the EF 24-70mm f/2.8 L USM lens, at The-Digital-Picture.com