The current de-facto language is C. I personally believe (as the creator of a JS IoT Library) that we will see a split in how we define the hardware field which will be a deciding factor in what things are written in.
What does 'robotics' mean? Is a device that waters plants a 'robot'? What about an autonomous drone?
Let's go with a very broad example look at any hardware controlling device which is not just sensing (so interacting with it's environment). Is that fair?
I believe that if a robot is doing dangerous and mission critical work, work that is maybe time and computer intensive, definitely needs a real-time clock for finite control, etc. etc. Than C is the way to go (happy to hear alternatives here, but I think you want to get as close to the metal as possible).
Thing is, C programmers aren't exactly a dime a dozen, they are fairly specialized. How many websites and apps are being written in C?
So, if the field is going to grow at the forcasted rate, there is going to have to be a huge volume of developers learning C to program IoT and robotics. But, how can you scale up that many devs. Or repurpose devs from the most popular language already, and one that runs on many low-powered devices. I don't think another language fits that mold as well as javascript.
For this reason, I've been building an IoT javascript library http://getfavor.net. Still in early stages, but keen to hear feedback.