People have already covered the main points.

It’s more so a combination of phone plan affordability, no universal plans that spread across multiple countries that are affordable. Where as Internet and wifi spots are abundant.

I know from AU and USA they all have unlimited text plans and very generous data plans.

Europe and probably other countries around Asia have roaming charges to chat to their local countries. Economic based choices to use internet messaging services because it’s cheaper.

From as far as I know people who use what’s app and telegram are from Europe or countries where it was more established for the mentioned reason.

Travel agencies use those apps because it’s a universal number for traveler’s to contact them without a sim. Or people use it for anonymity for drug purchasing.

We are sort of isolated from those parts of the world. Idk about UK but probably falls under satisfactory mobile plans.