They probably avoided Monero to not attract the additional scrutiny. They don't even accept donations in Monero.
Signal is not robust for metadata protection. Neither do they advertise anonymity. They take steps to protect metadata but it's nothing compared to SimpleX.
If it's "the feds", then how? There's reproducible builds on all platforms except iOS so we know the source code is what's running on our devices. Can you point to the code where the E2EE is compromised?
They are the largest messenger that has E2EE backups by default.