Why does this magnetic relay max DC voltage rating is much less than its max AC voltage rating?
Because one of the key features of a relay is its ability to interrupt a current.
But when the contacts begin to separate, there is inevitably a small arc that forms, especially if there is any inductance in the circuit being switched. It is important that this arc be &quot;quenched&quot; (stopped) quickly, and this is more difficult at higher voltages.
With AC power, the voltage and the current drops to zero twice each cycle,1 which goes a long way toward stopping the arc. But with DC, there are no such interruptions, which allows the arc to last much longer. And if the contacts don't separate by enough distance, the arc might not get quenched at all.
Therefore, any given relay design will have a much higher voltage rating for AC than for DC.
See also:

Using an AC relay for DC switching, safe limits
relay power ratings - AC vs DC
Purchasing a Switch - AC vs DC Current Specifications?
Exceeding Relay contact voltage

... and probably many others.

1 Not necessarily at the same time, if you're switching a circuit that has any reactance.