The good answer: They can't verify the integrity of alternative clients and that they don't leak info
The other answer: They've got somewhat of a "we know best" vibe going for them which also comes in play when you see their response to feature requests - e.g. for usernames instead of phone numbers or for "edit message" functionality like Telegram has.
I don't know what response to username feature requests you're talking about, but AFAIK they've been saying for a long time already that they're working on that — but it's a significant rework of their architecture, so it understandably is taking a long time, especially to also do so in a privacy preserving way.
>The good answer: They can't verify the integrity of alternative clients and that they don't leak info
The good answer isn't even a good argument.
The client is open source. People have forked it and used the Signal network. Signal asked them to stop and they did, but there is nothing stopping people from ignoring Signal's request in the future.
This has nothing to do with federation. Signal could federate their network and still request everybody use the official client.
It's a non profit so it can receive donations, but the developer is a LLC that's run for profit. It's a similar story in almost all software companies that market themselves as non-profit foundations (Mozilla too btw)
The effect is the same as it is for "regular" non-profits: there are no shareholders (other than the non-profit), and so no incentives to maximise profits.
(Of course employees and board members can still receive handsome compensation, but the same holds true for regular non-profits.)
There are some things non-profits are not allowed to do. But, owning a for-profit isn't one of them, and the for-profit is allowed to do those things. So hence this is a common strategy.
Example: Suppose I bulk buy T-shirts printed with my cool logo for $15 each and I sell them to consumers for $50 each. That's a for-profit activity, if Walmart was allowed to have a "non-profit" arm which did this I'm sure they would, the tax saving would be considerable.
That's decidedly not what "nonprofit" means. A nonprofit can absolutely fundraise like that, and pay the employees above market salaries, or they can use the funds for other things, they just can't return them to shareholders (of which there aren't any), or the board.
Signal also has a grand total of 40 employees. Keeping the application running on the various platforms takes a decent chunk of their time. Difficult development to add something like federation cannot be expected to happen quickly.
