Von Koz
Freaked Out
My reasons for losing interest are threefold:
1) I found Guns of the Patriots to be enormously disappointing.
2) I completed the first three games so many times that they became predictable and boring.
3) I grew tired of Kojima's implementation of sophomoric humour.
The Phantom Pain looks excellent, though, despite continued use of Kojima's... unique comedic style. The first two games featured the perfect ratio of absurd humour and seriousness. I especially loved Sons of Liberty.
You summed up my feelings on MGS pretty well!