Cost.
Also blitz / bughouse can damage pieces. Kids slam them down hard and sometimes pieces go flying, get stepped on, etc.
Some adults have really nice wooden sets though... of course you'd have to be careful they don't get stolen if you're traveling with something nice like that.
Plastic pieces are generally acceptable, vinyl roll-ups much less so.
Wooden piece sets usually go well with wooden boards, and those are a mess to carry with you on tournaments (in the US, many tournaments expect players to bring their own boards and sets). A vinyl roll-up board with plastic pieces is pretty much a standard, it is very cheap to buy and replace, easy to carry with you, and is just easier to play with, because people are used to them.
Instead of a wooden ones