G'Kar: Narns, humans, Centauri, we all do what we do for the same reason: because it seemed like a good idea at the time.
I presume that otherwise Boeing did a bunch of cost estimates and internal proposals and had a large number of discussions with current and potential customers and decided to build what turned out to be the best selling passenger aircraft on the planet. So the next question is, why did those customers buy it? I look at SouthWest, a company that not only weathered spiking fuel prices but seemed to get stronger and think, they know what they are doing and they have been 100% 737s. I don't know how they decided that, but I expect they also did a large number of estimates. It's very possible that the economics behind the decision won't ever be public as it represents an obvious competitive advantage.