The ecosystem, roughly speaking was: * Theano: Verbosity nightmare * Torch: Not-user friendly * Lasagne: A complex attraction on top of Theano. * Caffe: No flexibility at all, anything not the traditional architectures would be hard to implement * Tensor Flow: Unnecessarily complex API and no debuggability
I do not say that Keras solved all those things right away, but honestly, until just the fact that you could implement some Deep Learning architecture in 2017 on top of Keras I believe was one of the critical moments in Deep Learning history.
Of course today people have different preferences and I understand why PyTorch had its leap, but Keras was in my opinion the best piece of software back in the day to work with Deep Learning.