Mainly because of the screen dimensions of most phones. I think the optimal amount is in the range of 4-7 as you point out, but it didn't look quite as nice.

You can have less, but it encourages you to keep adding until you have six tasks.

It came from my own needs where there's a few set tasks I need to do every day in my own business. It's not that I forget, but sometimes I'm lazy, and this motivates me to ensure they all get done, each day.