I read a discussion of this issue and various media organizations have published about it. The one I read highlighted the evolution of the 737 and the addition of the CFM International LEAP engine, which is larger than previous generations.A TV talking head (sorry no link) says this is a consequence of the 737 (all 737s) having so little ground clearance.
Ref: http://theconversation.com/boeing-737-max-after-two-fatal-crashes-an-expert-explains-the-issues-113833So while all 737s are certified as a single aeroplane type, the changes have been massive.
Most recently, the big push has been to reduce fuel consumption. For both economic and environmental reasons, this led to the latest 737 MAX 8 models – along with the competitor Airbus A320 Neo – being fitted with new, larger and more efficient CFM Leap engines. These must be mounted higher and further forward than previously, creating a handling problem that wasn’t unique to this aeroplane, but had to be addressed.
https://www.businessinsider.com/boeing-737-max-design-pushed-to-limit-2019-3 (too much advertising)
The Wall Street Journal has published an article:
U.S. Federal Investigators Probe Boeing 737 MAX Development Choices
Probes to focus on stall-prevention system and examine whether any shortcuts compromised safety