Because UK is a vibrant and sophisticated democracy and before anything is done, there has to be the usual public debate(s) about it.
On average it takes about ten (10) years for the British people to make up their minds about any new development, be it a road or a railway or any such addition to an already packed infrastructure.
Take for example HS2 (High Speed Rail Link) this next link is part of the public debate - there will be no HS2 until at least 2026.
http://www.rail.co.uk/rail-news/2011/the-great-debate/
Cross Rail Examination Debate
http://69.89.27.212/~kempners/cross_examination_debate.htm
It took us not decades, but more than a century of argument to finally agree to a Channel Tunnel. We could quite easily have asked Mr Brunel to build such a tunnel - he had a band new tunnel making machine and built the Blackwall Tunnel here in London with it - a sort of test version if you like. But no. We don't want the Frogs just marching through to attack now do we? No sir.
If you ask the average Briton what they really, really want in terms of trains and a railway, you'll be stunned to discover the truth about us. Here's a sample.
https://www.youtube.com/watch?v=K4J7BEFnzQ8
The Greeks have a word for it 'nostalgia'. I was born in 1941 and as a boy saw many fabulous steam locomotives. We just cannot give them up.
Mallard - she holds the world steam speed record - we took it from the Germans and they're never ever going to get it back.
Mallard - bloody awesome -126 mph
https://www.youtube.com/watch?v=9sjXC65e-xQ
A4 high tail
https://www.youtube.com/watch?v=pFTphTnv2Q8
You can have any damn high speed train you like - they are plastic and have no romance what-so-ever.
The mighty Oliver Cromwell - full stream up
https://www.youtube.com/watch?v=vAXhjKNcKPQ
The Mighty Union Pacific Railroad Company - thousands gather to see their working steam locos.
https://www.youtube.com/watch?v=EhbCQ85nYNM
https://www.youtube.com/watch?v=ZjWBw4tZUO4
https://www.youtube.com/watch?v=Pi7Jac3htts
If the above did not excite you, then you have no heart.
And in case you are wondering, yes, I have been on a modern high speed train - every time I go to Paris - London St.Pancras International to Garde du Nord Paris 2.15 hrs.
http://www.bing.com/videos/search?q=eurostar+compilation&view=detail&mid=6C8BBBB988311B74D14C6C8BBBB988311B74D14C&first=0&FORM=NVPFVR
London UK 250213.1649