It's a major pain in the ass because every browser (hello Internet Explorer) implements a different set of features, and the features all browsers have in common, the browsers implement differently. And this is not only cross-browser. I've seen Firefox behave different under Windows and Linux, for example.

Then, there is a big performance difference between all the browsers, which means you have to strip the application down and optimize to the slowest browser.

Compared to the development of a native mobile application this is quite a big difference.