Why is the Active/Real Power Rating So Low for This Relay?
The difference is AC vs DC.

For DC, the rated voltage is only 30V, which at 10A comes to a 300W max load.

For AC, the rated voltage is 250V, which at 10A comes to a 2500VA max load. Note that this is Volt-Amps which for a pure resistive load is equivalent to Watts.

The datasheet is not saying that for AC, you can have 2500VA active power, but only 300W resistive power. Rather it is giving two specifications, one for AC and one for DC.

I've tried to highlight how these specifications link together in the annotated diagram below.