Worse is a strong word. I think all the mainline FFs are good games.
I just think at the end of the day XIII had a better battle system, a stronger presentation, was more consistent in what it was, a better(well maybe better used) soundtrack, and an interesting premise that it did more with.
I like XV. I think it's combat is good, walking around the world was great, the road trip angle felt fresh and well done, the cast was great, and the story had some cool ideas.
I just think in the end the presentation was a downgrade from the previous games, the soundtrack was poorly implemented or maybe not as memorable, the 2nd half is a mess, and the story is poorly told which is disappointing because all the elements are there.
I rank the FFs like this.
1.X
2.XII
3.VI
4.XIII
5.VIII
6.XIV
7.VII
8.XV
9.IX
and then the rest.