They don’t allow 3rd party clients, as per their ToS:

You must not (or assist others to) access, use, modify, distribute, transfer, or exploit our Services in unauthorized manners, or in ways that harm Signal, our Services, or systems. For example you must not (a) gain or try to gain unauthorized access to our Services or systems; (b) disrupt the integrity or performance of our Services; © create accounts for our Services through unauthorized or automated means; (d) collect information about our users in any unauthorized manner; or (e) sell, rent, or charge for our Services.

You need authorization to access Signal servers, which they don’t give:

we really don’t want forked versions of the app maintained by other parties connecting to our servers. Not only could the users using the forked version have a subpar experience, but the people they’re talking to (using official clients) could also have a subpar experience (for example, an official client could try to send a new kind of message that the fork, having fallen out of date, doesn’t support). I know you say you’d advocate for a build expiry, but you know how things go. Of course you have our full support if you’d like to fork Signal, name it something else, and use your own servers.

In my opinion, this is a horrible decision from Signal.