Because the two gauges are measuring different stages of the same problem. On a pressurised vessel, the pressure gauge responds to the rate at which liquid is turning to gas. The level gauge responds to how much liquid has already gone. Vacuum degradation shows up in the first long before it shows up in the second.
The practical consequence is that a vessel losing its vacuum will hold pressure higher than usual, or recover pressure faster after a withdrawal, while the level still looks acceptable. By the time the level gauge is telling you something is wrong, you have been losing nitrogen at an elevated rate for weeks.
This is why a daily pressure reading, logged rather than glanced at, is worth more than any single inspection. What you are looking for is not an absolute number but a change in the pattern at unchanged workload: pressure sitting higher than it used to, or building back faster than it used to.
Frost on the outer shell is the last warning, not the first. By that point the vacuum has degraded far enough for heat to reach the outer surface, and the vessel usually needs to come out of service. The pressure trend gives you months of notice. The level gauge gives you weeks. The frost gives you none.
