[witrak]

Because to check email your active participation is needed so you must start an application while to accept texting no such action is needed. Thus for short information (read-and-forget) short messaging (SMS or RCS) is more convenient. Of course, you could use a specialized application to check your email account and filter those needing immediate notification, but if such a service is already available what is the reason to generate additional much bigger traffic?

[somenameforme]

I don't understand where you're coming from here. Both Android and iPhone automatically and passively check emails simulating real time connectivity akin to a messenger. What you said does apply to desktop, but comparing SMS to desktop email is rather odd to say the least.