wont a fork work just as well?
Nope. A fork has only 4 tines whereas the cappings scratcher has about 20. That's about 1 inch versus at least four inches. One is four times more efficient at scratching the capped honey. By the way don't scratch the cappings, slide the scratcher under the capping with the tines running parallel, but under the capping and lift them off. It's more effective and less destructive to the combs.
The capping scratcher has thin tines whereas a forks has tapered tines. This is important if you are reviewing drones in pupal stages for varoa. You could go with a fork but it's pennywise pound foolish.
however, if yours goes missing at extraction time, as mine did, a fork will do in a pinch :-D