It's not that the pipefish/seahorse can't take warmer waters, as indeed, most come from water warmer than 74°, it's because the biggest nemesis of seahorses, bacteria (like vibriosis for example) multiply exponentially with each rising degree temperature of the water.
It's also not that there aren't syngnathids kept in tanks at those temperature.
It's the fact that for every one successfully kept at higher temperature, many many more die in the attempt to do so.
Your odds are immeasurably increased by keeping the tank water in the 68° to 74°F range.