I thought I had read (years ago) that because XM hardware is preprogrammed to receive Channel 1 without a subscription, that meant SXM was stuck using 1 for the "Preview" loop which these days usually sounds like Hits 1 mixed with lots of ads for the paid subscription service. So if they put the commercial free Hits 1 on channel 1, they either wouldn't be able to block it for those who don't pay, or it would cause other issues. (I'm sure, for example, that a lot of FAQs and support documents are written that tell people to tune into channel 1 to receive an authorization signal.)
On the Sirius hardware platform, the Preview loop has been on channel 184.