I suspect because the BT product, while "unlimited", continues to hide the details of the traffic management that gets applied.
Plusnet, on the other hand, are totally up front about their traffic management.
So when there is a combination of Plusnet products that you can buy, that has both a documented unlimited component *and* a documented "no traffic management" component (*), it's worth at least considering.
(*) - that's no rate limiting, but the traffic still has to follow prioritisation rules. And there's still management going on in the event of something biblical happening.