80% active is a pretty good score actually. But for whales who were early to invest in altcoins, Steem would be one of dozens of coins they own, while they might not be aware of Hive at all. Voting, even setting up automatic voting, would distract them from sipping cocktails on the beach. 

On the other hand, a few users in this list have been very active in the past, e.g. as witnesses, and may have gotten tired.