It's not laziness, it's cost to cost-sensitive customers who have a viable alternative with Airbus A320 Neo. Boeing actually initially set out to make an entirely new plane. The airliners instead wanted a re-engined 737 to keep type certification so that they didn't have to retrain pilots and could keep new equipment purchases to a minimum.

It's not that Boeing would have been unwilling to build an entirely new plane, their CEO announced that as the plan in 2011, it's that operators would rather they didn't.