Reason is severly banal... insufficient clearence between PL2 and CL1, which was easier to correct by cutting off half an inch of PL2 handgrip than by repositioning PLs and redesigning the entire throttle quadrant. In other words, originally there was no room for F/O's LH thumb. Difference is visible here:
http://www.!!!!!!!!!!!!!!/open.file/0706707/L/
BTW, ATR has plenty of rudder and there's no noticable difference in controllability during EFATO no matter which donkey quits (in a sim, anyway). Also all systems can be powered from either engine, therefore there is actually no critical engine on ATR. Also single engine taxiing is done on eng no2 and I have certainly never engaged prop brake during taxiing, nor I can think of any reason why should anyone want to do it. Both PLs have the go-around button, no1 for capt's thumb, no2 for F/Os.