Aside from that, though... I think it's just that it feels like an additional step. You want to get going -- especially if you're the sort of person who moves quickly through airports -- and now you're just waiting as the lounge fills up. After a long international flight, you don't want to be sitting and waiting.
I won't be sorry to see the mobile lounges go. They were a clever idea at the time, but they're hardly "lounges" these days. They're just buses, at a time when you really, really don't want to be taking a bus.