What could stop it from starting?

Hello,

You could have a faulty crankshaft position sensor.
The crank position sensor detects the crank shaft rotation and sends that information to the ECM.
If the crank sensor is faulty it will send incorrect signal to the ECM and the engine will not start. You will get it to crank but it will not run.
Over time with heat and vibration from the engine the crank sensor can go bad and produce starting or stalling issues.

You might find these articles helpful:

https://www.2carpros.com/articles/how-a-crank-shaft-angle-sensor-works
https://www.2carpros.com/articles/symptoms-of-a-bad-crankshaft-sensor
https://www.2carpros.com/articles/crankshaft-angle-sensor-replacement

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.