The reason there are “like 8 frigates” and only 2 destroyers has to do with the history of EVE.
(Disclaimer: I wasn’t there when EVE launched but I’m a 10-year veteran of the game, so most of this is “IIRC” territory. Treat it as such.)
In the beginning there were only Frigates, Cruisers and Battleships (and industrials, but that’s not quite related to the topic). There were no Destroyers and no Battlecruisers, those classes (and others) came a few years later. For argument’s sake we’ll call them Size 1, Size 2 and Size 3 (with Industrials also being Size 2, but still a seperate class.)
More than that, there were 6 frigates for all but the Amarr, who only got 5. The Magnate was considered “special” and was not made available to Amarrian peasant capsuleers until later. Each of these frigates had a basic role in mind, but were otherwise considered versatile ships in their own right. The same applied to Cruisers and Battleships.
Then Tech 2 ships started to appear, based off the T1 hulls. These are specialized ships with a very specific purpose, and from a game design perspective they were intended to “plug the holes” that were visible in the range of EVE ships. Around that time CCP also noticed that there was room for further ship classes, so we got a range of Size 1.5 ships called Destroyers, and a range of Size 2.5 ships called Battlecruisers. (Even later we’d get Size 4 ships, Capital Ships.)
And all these new ships with new abilities, so there was less need to make more ships. Destroyers only had one ship for many many years, the second Destroyer is actually quite a recent addition (by my books). At the end of the day these two “sub-classes” are an afterthought… and it looks like they always will be.
That’s the reason for not only the question of why there aren’t any Faction Exploration Destroyers, but also every similar question about why there isn’t Ship Type X in EVE: There’s currently no visible need.
There have been debates and discussions about adding new ships into EVE for quite some time, but only a few of them have become reality. I still remember the discussions about Flagships, for example - really decked-out T2 Battleships that use the racial BS hulls that aren’t either Black Ops or Marauders. I recall that they’d give additional leadership and command bonuses, but would also come with a heavy penalty if destroyed.
There’s also room for a Size 3.5 ship class - ships that are almost Capital Ships, but aren’t.
If you want CCP to add faction destroyers, make a good case for them. Look at the current line-up of faction ships and see if there’s room for improvement beyond the old argument of “moar ships!!!11” Maybe a Guristas Destroyer with a 25m Drone bay that gives good bonuses to Heavy Drones (like speed) so you’d get a small ship with one mean little sidekick. Or an Angel Destroyer that can actually fit a full set of 8 arties without breaking a sweat and gimping itself in other regards.
Don’t just sit there and whine. Make a suggestion with content. Contribute to EVE.