Its high ISP due to the lowest possible molecular mass and its enormously huge heat capacity.
It can't be competed by any other coolant, and that's the case when the coolant is at same time the fuel.
The historical HSHL SSTO projects of 1960s-70s include a plane with turbojets, a scramjet, and air scoops.
On start the spaceplane has its oxytanks empty, so weights much less. While the hydrotank is full.
It starts by turbojets using the hydrogen and atmospheric air.
Reaching several machs, it switches off the turbojets and engages the scramjet working on the hydrogen and atmospheric air, too.
It begins accelerating and slowly raising, taking air from scoops, liquifying it, and storing the oxygen in the oxytanks.
The lifting forse supports it, so it gets much heavier than on start.
It reaches the thin air altitude simultaneously with full oxytanks.
At the same time it starts feeding the engine with the extracted oxygen .
It closes the air intakes, and its hybrid scramjet is fed by the same hydrogen and the oxygen from tanks.
It reaches the orbit.
The hydrogen is used to cool both engine and hull.