I was going to write a long-winded essay, but instead opted for a bit of carpentry. This program lets you change two key numbers in Drop7: the 7 itself, so central to the game, and the number of turns between block advancement.
My hope is that, by playing with these parameters, you'll get a better sense of why 7 and not 6, or 5.
TECHNICAL NOTE: I was originally going to write my own version of Drop7 in JavaScript to test this idea. But thankfully, Tony Chang had already written it. All I had to do was modify the code a little, and expose some parameters. Thanks Tony!