There is one single reason behind it. It's ubiquitous. You can work for anyone and do amazing things if you're using JavaScript. You can do it instantly on any machine.
Not every company is a Ruby shop. Not every company is a .NET shop. It's my belief that 99% of the time both of these companies will need someone who knows JavaScript, and knows it well. Microsoft, Facebook, Apple and Google all use JavaScript to make great things.
And as a startup founder, if you're building any sort of web startup, some amount of javascript will be essential. Once you've learned the basics, one of the best book resources I've found, to take things to the next level, is Javascript, the Good Parts. For really advanced Javascript hackery, John Resig's Advanced Javascript presentation is great starting point too.
