Sadly no clue. I know it's quite a recent change (last couple of years), so not a result of the Smiler incident, or any other theme park accident that I'm aware of.
It could just be a 'Stealth-like' scenario where they choose to rotate the annual maintenance of cars. So they have 7 cars available, with the 8th being serviced / used as a quick spare part replacement, a la Stealth, where the 3rd train is always being serviced. But again, I stress I have no idea.
Definitely cynical, in that this wouldn't have been the reason. The park would not have gone "Inferno is running too efficiently, never has a long queue queue to sell enough Fastrack, we need to slow it down". The departments in charge of Operations and Fastrack are completely different, and beyond communicating throughput numbers, will have little interaction with one another. And certainly the Fastrack department would not be able to ask them to slow down, nor would anyone higher up.
However, whatever the reason for the slowing in operations and reducing throughput over the years, it is certainly a by-product that Fastrack is more appealing. And I'm sure they're likely to sell more tickets as a result.
A recent change on bigger rides I've noticed is that staff now have headsets to communicate with each other, rather than the phones. This means they can be in constant communication with each other, and don't have to stop to do so. This has pros and cons, but there's now no longer this scenario of people walking over to a phone to have a goss with each other.