"One explanation is the app became too popular with Western citizens for the US intelligence community’s liking"

It makes sense. There's potential for a backdoor in signal downloaded from an app store. Reproducible builds are a thing but not possible for iOS afaik. Also, remember the Pegasus is a thing. If you're really a target, having a device with a phone number is probably a bad move. For most plebs, Signal is "good enough".