Subsequent versions of IE have gotten dramatically better, but the time delta between versions is too long. From its wikipedia page:

    IE5: September 1997
    IE6: August 2001 (+4 years)
    IE7: October 2006 (+9 years)
    IE8: March 2009 (+11.5 years)

Safari, Firefox, et al do not have a similar lag and have been able to implement newer standards much more quickly. WebKit, for example, is in active development, and the version of the framework in use by Safari is updated fairly regularly.