Use of the word primarily suggests there might be other reasons as well which were not mentioned.
Thing is, though, charge-backs are pretty hard for a customer to do unless the customer is paying with Paypal which is why MA didn't allow Paypal payments until recently. I don't think the % of debit/credit card charge-backs is significant. How come Blizzard found a way that worked well? They also allowed credit/debit card deposits. I am pretty sure they were aware of the charge-backs risk as well.
The max limit for ex cash card holders was $3k USD/month and it was processed in 3-4 working days.
Also, if the primary reason is debit/credit card deposit/withdrawal risk, why not allow all wire transferred deposits to be withdrawn faster? Offer a special incentive to all people that deposit via wire transfer to withdraw the money in 3-4 working days. There's zero risk for MA when it comes to wire transfers. They offer similar feature for Neteller deposits/withdrawals. You can withdraw via Neteller the amount deposited via Neteller or something like that.
https://account.entropiauniverse.com/account/withdrawals/