Yeah, because that's where all the money is. The 'rich' don't actually have that much of the wealth in this county. Sure the top per cent or so are a couple orders of magnitude richer - but most of the wealth is with the middle 50%.
The problem with higher tax rates for the very wealthy is that is doesn't actually raise much money in the grand scheme of things, 'cos they simply don't represent a very large proportion of the nation's wealth.
As far as income tax goes the top 1% of earners pay 25% of all income tax and the top 10% of earners pay 50% of all income tax.