cost. Airbus planes cost more than the Boeing planes. That's why I almost always choose 737s over the A320 family. Cheaper to lease and operate.
In some of the earlier variants, yes. But with the NGs the price difference is virtually identical (in many cases to the dollar identical) and often times actually favors the Airbus model. In the quote you posted I was specifically discussing the 739ER and the A321, in which case, as I mentioned, the Airbus model is actually significantly cheaper
to purchase upfront.
Even just an upgraded MTOW 739 to get it even comparable to the range of the A321 will cost you more and still not fly as far as the lowest-ranged base A321.