-How could such great Rangers like Tommy, Adam, and Billy allow Rito Revolto and a group of D-list monsters destroy the Thunder Zords?
-How could such great Rangers like Tommy, Adam, and Billy allow Rito to plant the Orb of Doom and enable Zedd to stage his coup on the planet?
-How could such great Rangers...
...You get the point.
Rangers arent superhuman, there flawed human characters who don't always get the job done. This is one of those instances where the villain won.
There's an explanation for everything...and in PR, you just don't see it that often, because previous seasons arent important when the attention is on the concurrent one. Venjix is a NETWORK, it fucked SPD's technology up, likely cut off their long-range transmission, isolated the entire planet, and got to work undisturbed. Let's not also forget Alpha was once capable of gaining access to Rita's "databank" in "Doomsday Part 2", so any kind of mystical source would have one too, and from there, the databank can be used against them.