Is there a shorter or L-shaped version of PowerPole connector? if not, is there an engineering reason why not?
They're built that way because they are optimized for 1. Power handling and 2. Cost.

The pins are sheet metal construction (folded from a flat sheet) and typically tin-plated copper. As such cannot rely on a solid core to improve conduction and better materials (e.g. gold) to reduce resistance. 

All they have to work with is surface area and since the diameter of the pin must remain small enough to retain strength (it's a rolled up sheet) yet large enough to be manipulated by conventional forming equipment, the only degree of freedom left is the length... ergo the longer pins.