Key point here. Highcharts' customers are companies and website builders that don't have, get, or take the time to wrap their heads around D3, they just need to chuck a graph or two on their site(s), then move on to the next thing. It's not idealistic, but it's the reality.
I'm personally surprised there's not a lot more commercial JS libraries like Highcharts out there, seems to me there's a big market for those.
FusionCharts (http://fusioncharts.com) is another successful commercial product for charts.
I'm in the not-enough-time-to-understand-d3 camp, but maybe that will change. In the meantime, I've found it easy to get something going fast with the highcharts tools.