Taxes are a really tricky subject. Tax the top earners enough to run the country, but not too much so they leave.
The amount the 50% tax band raises is not worth the effort to track and gather it. i.e. it is a waste of time. Accountants manage their income down to £150k. At 40% a person earning 150k a year is paying around £60,000 a year in taxes and NI. How much more should they pay?
http://www.bbc.co.uk/news/business-17465733