I like Guardians of the Galaxy, but it should NOT have replaced Tower of Terror. It didn't need to be replaced. Why didn't they just clone the Guardians coaster they're building down here in Florida and put it in there instead?
I'm sure the really uninteresting answer is the basic combination of timing, budget, and space.
1. Guardians of the Galaxy was at peak popularity so they wanted to open an attraction as quickly as possible to capitalize on that.
2. Redressing Tower of Terror allowed them to install the ride on the desired time table at a lower cost than building a brand new attraction. Remember the Epcot roller coaster does not exist yet and would be subject to all the same experimental construction and testing as that one will be.
3. Disneyland does not have the space that Disney World does and needs to be more creative with how it uses its real estate. The Tower of Terror was right on the boundary of where they were building Marvel Land. Redressing Tower of Terror allowed them to have the attraction up early, up cheaply, and up in a location that made geographic sense.
I love Tower of Terror and hate that it's closed. But the reason for doing it makes a lot of business sense. That's not satisfying but it's the truth.