Because people have security needs beyond Bitcoin.

Over the long run we expect cryptographic keys to become important in a variety of aspects of our lives, not just finance / assets.

We recognize that some people disagree with this perspective and are willing to cede that market segment to Bitcoin-only competitors.

I suspect a lot of folks aren't aware of my BitGo history where I went through the same process of transitioning the company from Bitcoin only to multi-asset. I wrote about some of my issues doing so at length, and I still stand by my position that Bitcoin is better designed. https://blog.lopp.net/the-challenges-of-building-ethereum-infrastructure/

But at the end of the day we're running a business, not a church.