Because the money raised to make it into a museum was not enough, according to ABC at least:
The Navy sold the vessel after rejecting a bid from the USS Kitty Hawk Veterans Association to convert the ship into a museum stationed at Long Beach, California. The association could only raise about half of the amount required to decontaminate, develop and maintain the ship as a museum.
As I understand it, the ship was built in an era when asbestos was liberally used. There's even a ($12M) lawsuit seemingly won by a former crew member, related to that. So you can see how the Navy might want to avoid possibly being held partially accountable if a not sufficiently decontaminated ship were to become a museum.
The Navy managed to get some money for one of these supercarriers
the USS Constellation was sold to International Shipbreaking, but at a much higher price, with the company paying the military arm $3 million.
And I'm not sure Fox News got that right, because another source says it was the Navy who paid the $3M.
“Under the contract, the company will be paid $3 million for the dismantling and recycling of the decommissioned aircraft carrier Constellation (CV 64),” according to the official U.S. Navy website.
I think all the rest were scrapped for 1 cent though (Ranger, Forrestal, Kennedy). Why that difference is probably more a question for economics SE, than here, I think.
Similarly, the French Clemenceau had a hard time finding a scrapper; India, despite its generally lower environmental standards, blocked the contract; ultimately that ship was dismantled in the UK. Again asbestos is mentioned as part of the problematic materials; approximately 900 tons on that ship, according to one estimate. There are some similar US Navy documents but they lack quantitative data, only saying stuff like "The engine rooms usually contain the most asbestos and, therefore, take the longest for asbestos removal to be complete."