It's got fuck all to do with popularity. My old man forked out for a HD TV, a Freesat HD box, but he never ever watches HD content. He mainly watches BBC 1 and 2, which are channels 101 and 102 on Freesat. He never goes beyond the first page to find BBC 1 HD (108) or BBC 2 HD (109). The popular channels are popular because they are on the first page of the EPG, even when better quality equivalents of those channels exist elsewhere in the EPG.
It's even worse for me on Sky. The first channels are 101 and 102, BBC 1 and 2. The HD versions are at 141 and 142, which is blessedly a relief - they used to be 141 and 169 IIRC.
Either way, if you're just paging through the channels, you have to go past 40-50 channels of shite to find it. OK, its not all shite, but it is all add filled till you reach 141.