Nothing inherently binding this to 2.4ghz, but it is the band for bluetooth, wifi zigbee/thread etc. Not in love with that as the bet for much of IoT because its pretty range constrained if you are working with power constrained devices...For higher bandwidth applications it makes lots of sense but for sensors and actuators where I am only moving a few bytes at a given time, I would rather have lower power and better range... and yes, they did miss an opportunity to make it open.
Lower frequencies offer longer range, which is nice compared to 2.4Ghz. Which means that 2.4Ghz is also going to have more attenuation, so you need a higher TX power draw to account for that. Basically, to get the same range, 2.4Ghz is going to need more power than 900Mhz would, but 2.4Ghz offers better transmission speed.
