WhatsApp became popular long before Meta bought it in 2014. Signal and Telegram both came late to the scene, both around the time of the WhatsApp acquisition. Whatsapp was simply in the right place at the right time with little competition, and a combination of network effects and Meta mostly leaving it alone make it hard to get enough traction for anything else
The US has more of an Apple-monoculture and apparently moved to unlimited SMS plans much earlier than Europe, so iMessage was able to fill the same niche
The US has more of an Apple-monoculture and apparently moved to unlimited SMS plans much earlier than Europe, so iMessage was able to fill the same niche