The 100 meter limitation comes from physics and the installed base of cabling. One hundred meters is a convenient length due to legacy specifications (10Base-T and 100Base-TX) since the installed cable base was already limited to 100 meters (CSMA/CD, as you pointed out, limited 10Base-T to 100 meters). You may have noticed that every twisted-pair standard strives to meet the 100 meter length for this reason.
The medium, twisted pairs of copper wire in this case, has certain properties that have inherent limitations due to electromagnetism. This is a large subject, but a simple search will yield a lot of results. Basically, you have external and internal noise that will degrade the signal. The longer the run, the worse it is, and this problem gets worse at higher bandwidths.
It becomes a tradeoff: it is better to specify a fixed length and increase the speed rather than keep the speed the same and increase the length.