They tried to do what Microstrategy was doing, but there was already plenty of other BTC treasury companies. By pivoting to Ethereum, they’ve managed to raise billions of dollars, which is a lot more than they would have gotten if they remained focused on Bitcoin.
This was the company chosen by VCs to be the Microstrategy of Ethereum. The founders only cared about what was going to make them the most money in the short term. If they had been offered hundreds of millions of dollars to become a Fartcoin treasury, they probably would have accepted that deal also.
They are late compares to other institutional investors in Bitcoin market and they saw potential of Ethereum with the RWA trend since latest years. In addition, they saw that if they choose Ethereum and invest money in ETH, they will not be a too late comer like if they do the same with investment in Bitcoin.
These reasons are biggest ones why Bitmine decided to go fully investment in Ethereum, it's not because they dislike Bitcoin or see Bitcoin is bad as they surely know that without Bitcoin success, Ethereum can not gain success. However, in ROI comparison between Ethereum and Bitcoin, as well as advantage of being early institutional investors, they realized that it's more realistic to become a Ethereum market maker, than with Bitcoin.