Honey I thought you'd never ask, here's my two bits in lay terms:

If I'd have to give one quick answer it would be memory latency. The fact is that memory and computational power have grown immensely over the years, but the time it takes to retrieve a bunch of data from the memory hasn't really improved at the same rate. Some quick math shows that the speed of light must be an issue. The solution to that is to create smaller devices, such as the SOCs (system on a chip) that we are starting to see the past few years.

In less technical words: The postal service is darn slow. Only a few days ago you figured out you needed something small to continue your work, and since then you've been waiting and idling. The roads are fantastic, it's just that there's a speed limit. The solution is to take all the villages and condense them into a city, shortening the distances.

There's a lot more to it than that, and that's just one of the issues on only a hardware level and only one of the solutions.