Corporate procurement practices and policies definitely come into play, as well.
With Linux, there isn't a consensus laptop model that everyone will request. One ends up with a lot of one off business cases, vendors, service contracts, etc.
With Mac, you have a consistent upgrade cycle, one service contact, and no fragmentation of OS distribution usage, etc.
TLDR;
It's easier to say "I want a macbook pro w/ cinema display..." and "we hired another developer, please re-order a mac dev setup", than any similar Linux setup.
With Linux, there isn't a consensus laptop model that everyone will request. One ends up with a lot of one off business cases, vendors, service contracts, etc.
With Mac, you have a consistent upgrade cycle, one service contact, and no fragmentation of OS distribution usage, etc.
TLDR; It's easier to say "I want a macbook pro w/ cinema display..." and "we hired another developer, please re-order a mac dev setup", than any similar Linux setup.