Because if your debit card is compromised, your entire checking account can be drained before you ever notice the problem. Yes, you will eventually get your money back, but not for a while. In the meantime, your account will be frozen and you will not be able to access your money or pay your bills. Trust me, you do not want to have to deal with that - not at home, and especially not on vacation.
Using a debit card is always risky, but it's even riskier when you're away from home. Partly, because you just aren't paying as much attention when you're on vacation, but also because thieves know that you aren't paying as much attention when you're on vacation.