Is this actually the case? I don't know anything about that segment of the computing industry, but I just assumed that FCP was the industry standard.
Unfortunately the botched introduction of FCP X sent Apple back a decade in this market. FCP X was too different from 7, and it didn't even support many of the workflows that pros needed. Since users had to learn new software anyway, a lot of editors went to Avid or Adobe (Premiere is quite good nowadays).
A lot of people jumped ship to Avid and Adobe rather than wait for FCP X to eventually improve. Convincing them to take another look at FCP X won't be easy.
Have a look at the current options Apple is putting out. If you need something that's cheaper than what Apple puts out, or something that's bleeding edge, then you're out of luck. This is especially true with video cards.
Apple used to offer the best value workstations on the market. They got lazy, or maybe the Chinese / Taiwanese started undercutting IBM, Dell, and HP; which forced prices down to a point where Apple wasn't interested.
And Apple doesn't have a very predictable upgrade cycle. If you're spending $5,000 on a machine, only to see the next version come out a few months later, it's not going to be very fun. Nor is holding off upgrades, because you just don't know if a new model is around the corner.
Video requires a heap of grunt. Apple wasn't offering machines with enough grunt, or cheap enough machines. They looked at the cost of switching, vs the cost of sticking with Apple, and a lot decided to switch.