Ow, I love it too. Esp on my laptops - with a touch pad, double clicking sucks.
The reason it is default was due to usability studies all saying single click is far easier for new users... Which is surely true, even after years and years behind both windows and Mac systems, my dad double-clicks menu items and forgets to double-click items in the file manager.
I'm surprised to this day that GNOME didn't pick up on this. The way it is solved in KDE is quite elegant imho - the selection marker is quite big with the large icons that are default.