It's related to EU regulations on use of radio frequencies. The regulation in question is to avoid the 'blocking' of the 2.4GHz spectrum in areas that have many 2.4GHz users.
DSM2 uses two 'fixed' frequencies of the 80 that are available in the 2.4GHz band (the Tx actually selects two clear frequencies when it powers up). If you had 40 DSM2 users then the entire 2.4GHz frequency is fully 'swamped' and no one else could use 2.4GHz. The chance of 40 RC flyers all being in range at the same time is very slim but 2.4GHz is for a vast range of uses, not just RC. If they all used a DSM2 'type' system that used a fixed frequency then the band could be swamped in high use areas like cities. This is why the EU regulations limit fixed frequency (non-frequency hopping) 2.4GHz systems like DSM2 to very low power short range applications.
Frequency hopping systems like DSMX and most others don't have this same problem because they constantly jump (many times per second) between the 80 available frequencies. Even if you have 100+ users on line, the fact that you constantly jump frequencies means that for at least for most of the time you will find a clear frequency, so the system still works.