Lots of reasons:

(1) You don’t want to be an early adopter of new wallet software. Use well-established and widely used open source software for your savings.

(2) Web wallets have historically been terrible at security.

(3) Does not support Lightning, which makes it unsuitable as a spending wallet.

(4) It is a terrible idea to link your identity to your coins.

(5) They may be starting as Bitcoin-only, but their marketing materials talk about “digital assets” - big red flag.

(6) Third parties are security holes - there is no guarantee that they will not be co-opted and you have no control over them pushing a malicious update.