It's because the wheels can turn the engine more easily and for longer in higher gears. This isn't intuitive until you realize that you've flipped the inputs and outputs of the transmission, and this inverts the gearing relationship. _Higher_ gears are better at multiplying relatively little input (wheelspin) into a lot of output (engine rotation).
You learn the same lesson (2nd gear starts) with motorcycles, which have much smaller batteries and fragile charging systems so the need to push-start is unfortunately common.
Early date with the now-wife, we ride a little way out of town and watch the sunset.
Then I realise in my consideration for the lady to guide her off the bike, I hadn't actually turned the ignition key off and the headlight had drained the battery.
Now a 2006 GSXR1000 idles in first gear at about 20km/hr (~12-14mph?). And a 100m quick-waddle found 1st no good for bump starting due to compression lockup...
Thankfully we'd stopped on a ridgeline and only another 300m away was the descent which allowed me to get to 40kmhr for a second gear bump start.
2nd gear lesson learned about this bike.
Date saved.