Speed is higher on 5GHz and 6GHz because the channels are wider. For every 10MHz of channel width you can get ~100mbps if there is no interference. 2.4GHz goes so far that it is pretty much impossible to get a channel with no interference and speeds suffer for it. 5GHz has much more bandwidth and lower range so you are much more likely to find a free channel to use unless you live in an apartment building or such. 6GHz has even larger bandwidth available and goes even less distance than 5GHz, though not much less, so finding a free channel should be even easier.