Why the number of neurons or convolutions chosen equal powers of two?
Deep Neural Networks are usually trained on GPUs to speed up training time. Using power of two for the network topology follows the same logic as using power of two for image textures in computer games.

The GPU can take advantage of optimizations related to efficiencies in working with powers of two. (see https://gamedev.stackexchange.com/questions/26187/why-are-textures-always-square-powers-of-two-what-if-they-arent)