I don't know for sure, but I assume it is because banks do not release credit card sales to sellers for a few days, and since etsy is not a bank, they probably have to wait for the money.
I get enough sales through paypal to buy supplies , so it is not a concern for me. Also, after a week or 2, you should be all set and getting deposits weekly.
I also do this fulltime, and I would suggest that you figure out a way to have money on hand. You have alot of sales, so you will still probably get enough through paypal to buy your supplies.