Heya,
I could be wrong, but I believe it's due to payment processing fees, where if they allowed too small a donation, it'd cost more to process it than the money that would be made.
Refer to this part of their FAQ: https://itch.io/docs/creators/payments#payment-processor-fees
A payment processor’s fee is typically comprised of a fixed amount around 30 cents and a percentage amount around 2.9%. Due to the fixed fee, the total percentage of the entire transaction the fee represents changes depending on the price of your product.
For example 30 cents of a 1 dollar payment is 30%, 30 cents of a 2 dollar payment is 15%, etc. You can avoid high payment processor fees by selling your work for a higher price.
For this reason we recommend people minimally sell their work at $2 to avoid giving up a large percentage of their earnings to the payment processor.