Bittrex's HIVE wallets were actually down for a really long time (many weeks) before the fork. They likely started to update their nodes when the new 1.24 version was tagged (you can start running the new version before the fork happens) and I don't know if it just took them really long or they were waiting until the fork happened to re-enable, but it was a big pain for the people who do use Bittrex for HIVE (like me, because I refuse to use Binance after the Steem takeover).

In any case, you're definitely right that Binance typically has way more volume which is very important, and it's great that they're back up now. Just wanted to point out that Bittrex's HIVE wallets were down for a long time.