Having worked in analytics at various tech companies including Fb, the reason is almost always just that app users have way higher usage rates and ad clickthrough rates. Of course if you know basic stats you will realize this is spurious correlation (social media addicts prefer apps, using an app doesn't make one a social media addict), but somehow management never cares.
Ads, tracking, and analytics. The usual, unfortunately.
Smartphones are now the primary computing device for 70-90% of people depending on what demographics you look at. Smartphones shipped last year 1.2 billion, desktops 70 million. That's about the long and short of it.
I'm sure it will have a web version at some point relatively soon. But of course you want a native app for a Twitter clone, most people use it on their phone!
They make much more money with apps, and non-technical people prefer finding an app to entering a URL (because they also don't know about bookmarks).