There was Stannis on it before. Why did Stannis leave it completly undefended after he left for wall is enterietly different thing.
Also, why it doesn't actually serve it's actual strategic purpose of denying access to Blackwater Bay, since Euron is apparently able to sail right in and out without any trouble.
Stannis left it undefended because the showwriters have an unhealthy obsession with fucking over Stannis because Stephen Dilane didn't cover for their writing in interviews
Also in-show reasoning, honestly no idea why it was completely empty. It seems unusual that Stannis would just abandon the whole island given how it could be defended by a small garrison and it was so strategically important to his continued naval dominance in the narrow sea. Moreover, with Stannis having seemingly left nothing, it makes no sense whatsoever why the Lannister forces made no attempt to stop Danaerys landing on Dragonstone, given how much panic the island caused the Crownlands with what few forces Stannis possessed. Cersei was so scared, she nearly murdered her children, threatened by a few thousand men in Dragonstone - yet left no defences behind to stop Danaerys arriving with a vastly larger force on her doorstep. Which is a shame too, given how if she had, Danaerys would have been killed on Dragonstone as she led the advance.
I just observed something kinda funny. In Euron's scene he disses the iron islands, calling them birdshit-covered rocks, yet two seconds later is boasting about the Iron Fleet.
I'm sure this has been laughed at forever ago, and in fact I think I do remember being perplexed by it last season - where the fuck did he get the wood for all those ships then?
My advice for the show is just to get drunk on a nice glass of red white wine. A plot point is a considerably rarer thing than a plot hole in this show. Euron is the kinda guy who won the Iron Islanders over with an argument that the Iron Islands made no mark on Westeros, despite the Iron Islanders being well known for having created Harrenhal as a monument to their greatness, in all of Westeros having once nearly attained total hegemony. Then his fleet, fully crewed by men loyal to him, all defected to Yara for reasons. Then he crafted a new fleet - one larger than his previous, out of cloth, rope and wood conjured from thin air. Then he sailed past Dragonstone somehow (??!!!), guarded by the combined Navies of Dany's Free City remnants and the Ironborn Navy into Kings Landing. Then sailed out.
All to make a marriage proposal in a leather jacket
Of course, my GoTmod-dominated mind just assumed they sailed down the Riverlands rivers because that is a thing the Ironborn can do there.
They clearly arrived from sea, and even if, there is no "Suez/Panama" connection between The Sunset Sea and The Narrow Sea, even if you take rivers into account. Unless Euron carried the fucking ships on his back all the way to Blackwater Bay (after passing the Reach, which is Tullys, which is currently Dany's allies), he can't get through.
The Ironborn could sail down the rivers if they used ships they constructed in the rivers, there is no connection between the sea and any of the Rivers in Riverlands which could possibly feed into King's Landing - and certainly no deep-hulled warships that could pass through into the bay (and then pass through Dragonstone on the exit). It doesn't matter, just resign to despair like Cersei. Show will never be good again, but at least we can still spoil the books before grmm does