According to ChainCatcher, Farside Investors said that ETHE's net outflow is higher than GBTC. One of the reasons is that GBTC was launched earlier than ETHE and holders are reluctant to sell it.

Another reason is that ETHE holders may want to stake their tokens. In addition, ETHE's fees are 2.5%, while GBTC is at least reduced to 1.5%.