It doesn't for the same reason there isn't already a "WeChat" of the west.
Additionally Twitter has as far as I can tell (speculative) a much much less stable foundation in the EU then it seems if you just look at daily users count.
EDIT: It's not only what everyone has (WeChat in China), instead of just many people (WhatsApp in EU). It's also the default payment service, you pay through it all the time in all kinds of situations. It's like WhatsApp but more monopolistic combined with PayPal but also more monopolistic. And both with full government support so no worries some bank lobby gets in you way.
I can't square that ambition with the idea that Twitter is 4x overstaffed and needs to shed 75% of its workforce, though.
If he has ambitions to turn Twitter into an Everything App, he'll need to diverrt a significant portion of the workforce onto that project, leaving him with a fraction of a fraction of the current workforce to keep Twitter's microblogging and ad shilling platform up and running.
Twitter is a pool of highly capable people, a large user base, and, currently, opportunity. A messaging platform added to their microblogging service is not outside the realm. It already has DM and Vine-like video.
He mentioned "the everything app" I think once? But -- IMHO -- people who tweet don't fit the profile of people who would need such an app. They're trying to sell stuff, not buy stuff.
They are also demonstrating that they're likely to use specialized services for specific tasks - Twitter is a very specialized tool. This is the exact opposite from someone who draws any profit from an "everything app".
If not even Facebook with its userbase of people already using a single mediocre tool for different purposes manages to crack the "everything app for the west" nut, Twitter is bound to fail.
Maybe everyone is bound to fail simply because the concept does not fly in the west.
Additionally Twitter has as far as I can tell (speculative) a much much less stable foundation in the EU then it seems if you just look at daily users count.