It seems like withdrawing from Utah is the obvious option

As the article explains - this was not an option.

>SB 73 burdens the rights of all internet users outside of Utah because it requires adult websites to either know every visiting user’s physical location, and then block those in Utah, or to verify every visitor’s age just in case they might be in Utah. The law’s “actual-location provision in practice requires an entity to perform age verification services for every user visiting its site from any location because the entity would violate the law if even one of those users happened to be obfuscating,” the court wrote.

In order to withdraw from Utah, they would be forced to block traffic from everywhere since it's possible that some kid in Utah used a VPN to make it look like they lived in Singapore.

That was what the law attempted to do: Ban porn in Utah.

There is a very vocal anti-porn group in Utah. They do things like put up massive billboards that say "[Store name] sells porn." (Which is basically free advertising instead of shaming.)