While I agree that Worldcons should maintain such facilities, there are people who think it's "unfair" to "force" Worldcons to have to have such things, and to force them to set up credit card/PayPal acceptance accounts for the benefit of another organization. Remember that the money paid for the ASM does not belong to Worldcon N, but to Worldcon N+2. Usually Worldcon N sets up a separate bank account in the name of N+2, but that generally doesn't cost any significant amount of money. Credit card accounts cost money on every transaction. Who pays the fees? There are a raft of practical implementation details that can derail the process.

As far as explaining goes -- I rarely mind doing that. The more people who understand how things work, the better. Informed voters and all that.