It's probably in part because of the fact that iOS isn't as big in Europe; outside of the Netherlands (where iOS controls about 70% of the smartphone market) it tends to be pretty equal in market share with Android and there's of course no iMessage on Android. People want to talk to each other, share photos and WhatsApp was cross-platform with Android basically from the start.
Telegram is a bit of a late arrival comparatively, and I couldn't tell you why it's big in East Europe, it just... is. Probably something to do with the remaining Russian influence in those countries?
It makes total sense why WhatsApp is designated a gatekeeper but iMessage isn't.