As spronrad notes, the problem was that at that time Boeing was severely resource limited. Remember, the 787 was supposed to be certified and in service in 2008. Not only did the delays on the 787 mean that engineering couldn't be released to work another program, Boeing had tens of billions tied up in program (at one point it was estimated that Boeing had over $30 billion just in inventory costs related to the 787 delays). The 747-8 program was also going full tilt - and also over budget and behind schedule although not nearly as bad as the 787 (and most of the schedule issues on the 747-8 could be traced to our being resource starved due to the demands of the 787). There was barely enough to do what needed to be done to get the 787 and 747-8 certified and into service - forget launching an all new program...
The original plan had been to have a new 737 replacement aircraft ready by 2016 (for a while the development program was called 'Project 2016') - which incidentally is when the first A320NEO was delivered. But the fiasco of the 787 program meant that Boeing simply didn't have the resources necessary to do that - which ultimately lead to the MAX fiasco.