Portability and heat. You can get a small USB 2.5G adapter that produces negligible heat, but a Thunderbolt 10G adapter is large and produces a substantial amount of heat.
I use 10G at home, but the adapter I throw into my laptop bag is a tiny 2.5G adapter.
I knew it runs hot before I deployed it, but I wasn't aware that you have to wait for it to cooldown before unplugging, or you get burnt.
There's of course fiber too...