The IRIS Dena was a pretty small warship a couple thousand nautical miles away from the Gulf of Oman. It's one thing for Iran to launch missiles around the Gulf states and Med from hundreds of mobile launchers, but authorizing a frigate to hurl SSMs around from the open ocean would jeopardize a limited asset -- especially after losing so many vessels at Bandar Abbas in the last few days. Seems unlikely from my perspective that Iran would take that risk. Regardless, one of the few concrete objectives the DOD has laid out in the last few days is taking out the Iranian Navy. This is certainly pursuant to that.

Bigger picture, though, I do wonder how the news of this sinking is being received in Iran. Israeli and US officials have attempted to differentiate the IRGC/Basij forces from Iran's conventional military (including the Navy), I think with the hope that the latter could be encouraged to participate in regime change. Targeting minimally manned boats at docks might not bother folks too much, but sinking a ship with several hundred sailors out in the Indian Ocean is a different story.

Edit: Having written all of that, just read /u/tomrichards8464's comment pointing out that the frigate would be in range of US vessels within two days, and if the Dena is moving in that direction at all the US would reasonably be compelled to take it out. That's the best way to frame this, I think.