There are many reasons not to use low frequencies such as below 30 MHz. Here are just a few:
1) It is prohibited by law. These frequencies are assigned to other services. 2.4GHz is a shared service... Even your microwave oven cooks there.
2) They need large antennas to function. The shortest antenna at 30 Mhz, which still is effective, is about 5 meters long (while at 2.4GHz it only needs 6-7 cm).
3) Shannon's limit isn't (a limit, I mean): the formula $C=B\log_2(1+\frac{S}{N})$ just says that you can send more info over the same bandwidth if you improve the $\frac{S}{N}$ ratio.
4) Then, you should realize that if you want to be at Shannon's C rate, you have to have the channel for yourself. No other users can use the same frequency. If you realize that 2.4 GHz is a jungle with probably 100s of simultaneous users...
So, if you want to improve your channel capacity (C), you should concentrate on improving the S/N ratio, which is much easier to do at 2.4 or 5.2 GHz than at 0-30 MHz