The temperature recommendation of 68° to 74°F (for tropicals) doesn't have much to do with the actual temperatures that seahorses live in in their natural habitat.
It has everything to do with the bacteria that are contained in our boxes of water that don't have the water constantly changing as in the wild.
In our tanks, the captive bacteria multiply exponentially with each rising degree temperature, especially as temperature rises above 74°F.
That doesn't mean that they can't be kept at warmer temperatures, but the fact is that many many more die at those higher temperatures than at the lower ranges.
IMO, seahorse keeping can be hard enough without making it any harder.