As a Signal user, you might have remembered when Signal/TextSecure supported SMS/MMS on Android in addition to Signal Protocol messages, which was a big factor in helping Signal onboard new Android users when it had a smaller user base. Signal never had the opportunity to do the same on iOS because Apple only allows its own Messages client to handle on-device SMS/MMS messages, which gave Messages (and iMessage) an unfair advantage over Signal (and the Signal Protocol).
Anticompetitive measures like Apple's SMS/MMS client restriction harm the market even when alternatives to Apple's products exist. The entire blue vs. green bubble issue would not be a problem in the first place had iOS users been able to switch to a competing messenger app that supports both SMS/MMS messaging and a newer cross-platform messaging protocol (instead of iMessage).