We chose Python for Theano because Python was already the language of choice for our research lab. If it had been my choice, I would probably have picked Scheme (I was really into macros at that time) or Ruby (I think it's better designed than Python). But if we had done it in another language than Python, frankly, I'm not sure it would have taken off in the first place. Python already had quite a bit of inertia, likely thanks to numpy and matplotlib.