Few creatures have Reactions, was this intentional design? Why?

@mikemearls So few creatures have Reactions other than the usual opp. attacks. Was this intentional design? If so, why?

— AndruC (@AndruC) August 17, 2017

Very much intentional – DMs tend to overlook them, saw they were hard to track in play. Lots going on behind the screen already. https://t.co/G5XuhZpHUF

— Mike Mearls (@mikemearls) August 17, 2017