1. Lighting conditions
2. Film exposure (based on your meter and metering technique)
3. Developer choice
4. Developing time
5. Developer temperature
6. Developer concentration/dilution
Some like to add film contrast into that equation, but that is a constant, not a variable, meaning you have to compensate for it with technique. Which is exactly what you found out by shortening your developing time.
If negative contrast is too high, you can mitigate that by exposing more to get the shadows off the toe of the curve, and then shorten your development time in order to give you highlights that print/scan well. Total negative contrast is always about the differential between the highest printable tones and the lowest. If they're too far apart, tonality is compromised by either blocked up highlights, or shadows that did not get enough exposure. Technique. Not materials.