Interesting

They are easy to understand but what was the logic behind the phonecian numbers?

It's all about angles !
It's the number of angles. If one writes the numbers down (see below) on a piece of paper in their older forms, one quickly sees why.
I have marked the angles with "o"s. No 1 has one angle. No 2 has two angles. No 3 has three angles. etc. and "O" has no angles