It was what I had. That, and I've always had better luck with glossy engine enamel than flat or semi-flat/gloss on my Jeep - seems more durable and easier to clean.
Again, mine is not a faithful restoration by any means, even though I'm trying to maintain some of the stock appearance. I'm going more for a "period correct" restomod: Cragar S/S rims, staggered tires, Lakewood traction bars, louvers, etc. I kind of wasn't paying attention when I did it, and painted my engine with Duplicolor Old Ford Blue, rather than Corporate Blue - and caught all sorts of Hell from "purists" on other forums for not using the correct paint. Honestly, I actually like the "Old Ford Blue" better than Corporate Blue anyway, which the negativity from the others just made me more resolute in the mindset of, "it's my car - I'll build it how I want it."
Short answer: The glossy black also just makes the engine bay "pop" a little more along all of my other engine bling.