For the same reasons we give our (rich) presidents a big white house and a retreat in the Maryland mountains and a huge blue jet, and in the past, a big yacht (until one of the idiot occupants of the office ordered it sold in the 70's for pretty much the same reason they sold off Britannia - trading a rich, grand, and graceful piece of history for the sake of a momentary PR gesture to appear frugal).
That ship was a grand, beautiful and living piece of British history (and frankly made the Sequoia look pretty sad in comparison). It may sound out of fashion in this Kardashian world but nations and people need and love institutions and images and ideals, and the Britannia was a piece of that image - and since this is a boat forum after all, I don't think anything captures a public's imagination quite like a graceful boat or ship. Look at the powerful hold the Titanic and the other grand liners still have on the public, even though they've been (mostly) replaced generations ago by gargantuan floating hotel and amusement park barges.