In this day and age, you can broadcast a full-scale radio station from your house, over the internet, so it doesn’t interfere with radio or television broadcasts - maybe not as spectacular as the professional ones - you’re its sole presenter - you could just stream continuous music during downtime and come on the air live as you find time! You could even play listeners’ requests!
I still have an old radio with "Radio Caroline" on the dial. Happy days from the 60's But Radio Luxenburg was the pioneer that I remember from the 50's - is it still going?
Because the highbrow toffs of the government and the Beeb didn't want any members of the general public to hear any kind of media output from any other source than their own, as they saw this as a threat. What music to listen to, what news to believe, the lot! Their method was (and still is to a large degree): Control, brainwash, conquer! Any outside source threatening that and gaining public attention would be severely frowned upon and potentially outlawed and destroyed.
In answer to your comment "No it wasn't" Putting it quite simply...the government of the day "Labour" wasn't earning any revenue pertaining to the advertising tax aspect regarding the pirate ships, under the pretext that the "airwaves" being used were a danger to shipping.
Why? Why not Tony Blackburn? Who knows more about commercial pirate radio than Tony having been at the very forefront of Pirate Radio at the time? Tony spent three years on stations Radio Caroline and BIG L Radio London so I would argue that, that is more than enough experience and expert knowledge to be able to administer to the public just what it like to be a Disc Jockey on those ships. He also of course helped to bring legel pop music radio to the United Kingdom by being chosen to open up BBC Radio One on September 30th 1967.