Some reasons I can think of.
1. If your keen on doing SETI (single engine taxi in), then it's better to turn off PACK2 after Landing. After cool down period you can shut off ENG2 and finish your taxi, but still have fresh air supplied to the cabin via Eng1->Pack1.
Otherwise you would have to open the cross-bleed to supply Pack2 with ENG1-Bleed.
2. as maybe 1% of my captains perform SETI, I usually just alternate between packs. That allows equal distribution of wear.
That is pack 1 for home base , pack 2 for away from home base
3. Noise for the loading crew can be a factor.
4. Business Class PAX should get more air. (Pack1 supplies more to forward cabin, Pack2 more aft)
Last edited by EDDT; 26th Oct 2016 at 21:43.
Reason: added no4