The premise of the question is wrong in the first place (considering the source, not terribly surprising). The ACA website is not a "simple website". In fact, it's extremely complex, and has to interface with many other disparate federal IT systems. The federal government puts up "simple" websites all the time.
And if you're looking for a reason why this fiasco happened in the first place, look no farther than the GOP-run states who, in a deliberate attempt to obstruct the law (likely an extension of their explicitly-stated intent to obstruct anything President Obama did), chose not to meet their responsibility under the law and put up state-run exchanges.
Funny -- usually conservatives LIKE it when things are left up to the states. I guess that premise goes out the window when a chance to undermine President Obama presents itself.