This is a backward argument; FR has delivered more because it has been the most published. That's not an argument for it being published even more, I'd argue that it's an argument for the opposite.
The second part, that there are a huge amount of FR fans who want this, is also not an argument. I can easily argue that Dark Sun fans want their own setting book, and Greyhawk fans want their own book (if the SCAG isn't a book for FR, Saltmarsh certainly isn't one for GH).
And every time someone posts a poll on "which setting should get a setting book next," Dark Sun is the runaway winner. So popularity doesn't support you entirely here.
It's been published more because it delivered, it sold better then other settings even when correcting for volume of content, otherwise other settings would have been published more, and it sold well across editions. You sell the settings that sell the best, it's that simple.
And of course Darksun has fans and same with the other traditional settings and they deserve their time in the sun too, it's just that their are a lot more Forgotten Realms fans then other settings (Exandia could evovle into an exception).
Even Forgotten Realms subsettings like Al Qadim and Kara Tur and Maztica are more popular then most traditional D&D settings.