IMO,
1. Student Body/Alumni Size - The ACC has 8 out of 18 schools that are private (the largest of which has 23,000 students - Syracuse). Even its public schools are on the smaller side - only 3 institutions with more than 40,000 students (FSU, GT, and Cal). The ACC has no schools with over 50k students. As a comparison, the B1G has only 2 private schools (NW & USC - USC has 47k students). B1G has 9 schools with over 50,000 students (including 2 over 60k).
2. B1G Much Better Business Decisions - B1G made the brilliant move to be the first major conference to start its own cable TV channel and has used its success to gain the upper hand financially over the rest of college sports to this day. B1G also smartly expanded based on football and led to the record-breaking TV deals they are now enjoying.
ACC on the other hand, expanded based on basketball and was late to the game in getting a TV network, and when they did, they signed away their fortunes for up to 20 years (depending on ESPN's decision in February 2025) on a ****** deal financially in the long run.
To add insult to injury, when the ACC DID expand based on football 20 years ago, Miami and VT dropped the ball on them and have been mediocre for the vast majority of their membership. And then when they inexplicably expanded based on basketball a little over 10 years ago, the schools they brought in - Syracuse and Pitt - have been mediocre in that sport as well.