There wasn't any particular reason, just an oversight. I've added a conversion path for HBD to BCH now.

Yes, you can send DOGE from a wallet to our site to buy HIVE. Do something similar to instructions here, but select DOGE as coin to send and Hive as coin to receive: https://blocktrades.us/en/support/home/help/trading
This will get you a DOGE deposit address where you can send DOGE from your wallet in order to buy the Hive.