Why can't I install apps on the iPhone 6?

iOS 12.5.7 is last security updates for iPhone 6, can’t be update to a higher iOS (13 and later).

Yes, most apps require iOS 13.4 or later.                                

Considering a newer type of iPhone?

Read this before you buy a newer type iPhone.

Probably because those apps require a higher version iOS than your phone is capable of running. Time to get a new (or newer) phone.