Because it hasn’t actually been dated.

The argument for the antiquity of the submerged section of Dwarka, so far as I can tell, is solely based on the fact that it’s under water. That land was last exposed 9000 years ago at the end of the last ice age, supporters of an old Dwarka say, so it must have been built before then.

The problem is that that’s not the only way it could have ended up under water. Consider this lion:

Or this stone head:

These statues are found just off the shore of Alexandria in Egypt, along with a great many submerged buildings. Does this mean there was a deeply prehistoric city at the mouth of the Nile as well?

No, it does not. The architecture and statues are solidly Hellenistic or later. They were constructed in a well-established historical period and ended up under water later, likely as a consequence of tidal waves (we’ve got textual evidence of that happening during the Classical era), perhaps helped along by soil subsidence, seeing as it’s built on the silt of the Nile delta. I’d note that Dwarka is, like Alexandria, at the mouth of a river, and that the Indian Ocean is prone to tsunamis far larger than anything you’d see in the relatively shallow Mediterranean. Basically, the same thing may very well have happened there.

So, then, we can’t rely on the fact that there are underwater structures alone to date the site. Archaeologists have to examine artifacts to see if they stylistically match artifacts of known historical or prehistoric periods, which will give us a date, or directly date artifacts with techniques like C-14 dating. A very old date hasn’t been definitively ruled out, so far as I know, but it does seem very unlikely.