It's probably a combination of factors.
Skype's removal was due to a complete change to the technology - it's only really the brand that's the same.
https://support.skype.com/en/faq/FA3...nger-supported
I think the Twitter app was written by Samsung, so it may just be that it's not used enough.
But I think as time goes on, it's clearer that some things just aren't suited to a TV, and it's better that the focus stays on those that are TV suitable. Social Media is difficult to use with a TV remote!
I'd point out that Samsung have added a Google Play Movies and TV app to my 2014 Samsung Smart TV, so it sometimes does go both ways.