The reason is just that it would be more risky, I think. Compare the scenarios:

1. Scammer clones your credit card with a skimmer and pays for $500 of clothes at the mall. You dispute the charges. The funds are actually not given to the store for a bit given that credit transactions take a while to settle. Upon the dispute, the store now needs to prove that you were there and bought those clothes to get their $500, or else the bank/Visa won't pay them.

2. Scammer clones your debit card with a skimmer and pays for $500 of clothes at the mall. You dispute the charges. The store already got paid though. The bank doesn't want to give you another $500 in case you are actually in on the scam, then they'll be out an additional $500. Eventually assuming they can't prove you actually bought the clothes, I think the store would have the $500 confiscated, but usually you're still liable for $50 if you reported it quickly enough, but could be more if you take too long to report the fraud.

Of course debit cards can easily be converted to even easier-to-launder money substitutes, too.