For the most part, the mechanics of the current bipartisan system.
In the current election system of primaries (in place since 1972), an incumbent has never lost. They haven't even lost a state in 44 years. So basically the incumbent has a defacto first right of refusal when it comes to their party's nomination.
The other party has been shaped by a political movement with a lot of power and momentum, and the founder of that movement more or less has the same defacto first right of refusal.
I think the parent comment is commenting on presidential candidate ages, which has had two consecutive new records in age and incumbency didn't seem to factor.
Prompt: list of us presidents and their year of birth and age when elected
You will see a list with few year of birth regressions and most ages in 50-60. Two years of 70+ is an outlier.
In the current election system of primaries (in place since 1972), an incumbent has never lost. They haven't even lost a state in 44 years. So basically the incumbent has a defacto first right of refusal when it comes to their party's nomination.
The other party has been shaped by a political movement with a lot of power and momentum, and the founder of that movement more or less has the same defacto first right of refusal.