Other contributors have said "How many commands do linux and unix share?" is the same question as "Why do you think Unix and Linux share more commands than Windows XP and Unix?". If you believe that these are not asking the same thing and should be answered differently, click here.
Because Linux was modeled after Unix, and designed to be a suitable free replacement. Windows is targeted at a slightly different market, and was primarily designed to be operated using a GUI, not the command line.