Simplicity. The Metric system has only 7 basic measures, plus a substantial number of measures using various combinations of these base measures. The imperial system (prior to the UK converting to metric) and the USA system have over 300 different measures (see
http://lamar.colostate.edu/~hillger/traditional.htm), of which many are ambiguous. For example, there are eight different definitions of ton (including the 'short ton' and the 'long ton'), all of which differ in weight. There are also eight different definitions of barrel, five different definitions of bushel, three different definitions of mile (international, nautical, and US Survey). The USA gallon is smaller than the Imperial gallon, and all measures which are multiples or subdivisions of the gallon (e.g. pint, quart) likewise are inconsistent between the USA and the UK. Not only are the USA and UK pints different in size, but there are also differences between 'dry pints' and 'liquid pints'.