You are right, few former Patriot receivers do well on other teams. Welker's case has been well discussed, and with Graham and Watson they just got more money elsewhere than the Pats would pay.
I guess you could argue that Hoyer and Mallett both were cut (or traded for negligible) because something better came along but both are still playing.
It is just weird to me that there are so many former Patriot defensive backs (Moore, Arrington, Butler, Merriweather,
Ihedigbo, and Brown) that were cut for performance (as opposed to getting a giant payday with another team like Revis, Talib, and Samuel) and ended up having productive career with other teams. Any Patsfans football brainiacs that can explain that?