Theory time!
Like players who are given different rating based on potential (hence different value between 2 players with same age and rating), clubs are put in different tiers too
Big clubs (in real life) like Arsenal, Chelsea, Liverpool are expected to challenge for titles every season, hence chairman will give them funds (cash injection) to achieve it
Manchester United (in SM) is the biggest club in this game world. Most titles, largest stadium, highest squad value, so chairman won't give anything extra
Other division 1 clubs may have stadium increase and/or cash injection depending on their success in SM (may be significant less than those real-life big clubs)
Small clubs (in real life) like MKD and Bristol City won't get anything from chairman even when they achieve promotion to Div 1 in SM. They may need to survive the drop for at least 1 season before they can get anything from chairman (hence SM comment about clubs getting financial boost if they continue to gain success)
But that's just my theory. A GS theory