Encryption would break compatibility with existing equipment. Users hate that and would see that as an opportunity to switch to another system.
Encryption will use more processing power on the receiver (rx). The rx gets warmer and uses more electricity.
If you want to break the encryption you can do so at your desktop with much more processing power than what is practical to use in a tiny rx. You will also be able to take advantage of developements in technology.
An encrypted system is harder to debug.
Encryption sometimes (depending on the implementation) increases frame length and therefore risk of frame loss.
The user gets all the drawbacks but does not benefit.
The benefit to the vendor is questionable (but that's a topic for another thread).
Are there alternatives?
1) Rather than encryptiong the actual communication the rx may send a digital signature with it's GUI, model ID, protocol ID, a random number already received from the transmitter etc. at bind time. The transmitter can check the signature at bind time and refuse binding if the signature does not match. This gets around some but not all of the above problems and may still be too easy to break.
2) If you introduce an inovative new feature in your protocol you could patent it. This may already have been done. But there are all kinds of trouble getting and enforcing a patent.