Because you will fold often to raise and lose chips, and if you have 20-25BB stack or less you don't have limp range. And especially in mid and late phase in the tournaments.
A common beginners error. You hold a speculative hand like Q Spade T Club or 9 Heart 8 Heart and you would like to take a look at the flop. The cheapest way to get there seems to be the limp (calling the big blind). That's wrong!
Limping might be tempting, but it doesn't make sense in most circumstances. Getting the hand started with an open raise is the way better play. That way you enable yourself to get a hold of the pot preflop without having to play further streets.
from lessons