Why has the progression of smartphones been so fast?

The title says it all. I have a speech I have to do for school and this is one of the points I have to cover and googling didn't do much for me.

What has enabled this fast progression technologically? For example when looking at the Galaxy and the S 4:

Galaxy S: 480x800 and 512mb RAM

Galaxy S4: 1080x1920 and 2GB RAM

All answers appreciated.