Because the Followed feed is purely chronological. An interesting tweet from someone I follow could have happened 3 hours before I opened the app, and I would miss it.
This is why I preferred the old For You tab - it was (mostly) the people I had chosen to follow, but meant that I had the best content show up whatever time of day I opened the app. This is particularly important when I'm in the UK and most of the people I follow are in the US, so they're not tweeting generally at the same time I'm on the app.
On the android app at least, a recent update made it so pressing the home button while you're in the "followed" tab switches to the "for you" tab. It's extremely annoying
This is why I preferred the old For You tab - it was (mostly) the people I had chosen to follow, but meant that I had the best content show up whatever time of day I opened the app. This is particularly important when I'm in the UK and most of the people I follow are in the US, so they're not tweeting generally at the same time I'm on the app.