Because other payments are monthly (rent, ISP, mobile phone contract, subscriptions - at least in Europe), so having a constant money in <-> money out equilibrium makes sense. Unless these things are also charged every 2 weeks in the US.
I think the idea in the US is that if you're having to wait an extra two weeks to get paid, you're essentially having to "float" half your monthly income all the time. This impacts people who are on the poorer end of the scale, and there's a very high percentage of Americans that don't have more than a few hundred dollars to their name.
Plenty of US places pay twice a month, so you get the monthly budget consistency and get your money sooner.