This might possibly be a better question asked of physics.SE, but the problem, uh, boils down to efficiency of heating.
Firstly, it is generally easier to heat a liquid than a gas. Loosely speaking, this is because the thermal conductivity of gasses tends to be lower as a result of their lower density. Liquid water, for example, has more than 10 times the thermal conductivity of steam. You can get the same mass of liquid and a gas to the same temperature, of course, but it can take longer to heat up that gas.
Secondly, the efficiency of a heat engine is proportional to the temperature difference between the hot end (the boiler) and the cold end (the condenser). You want that hot end to be as hot as is practical.
Finally, your engine delivers power by basically moving energy from the hot end to the cold end. If it takes too long to heat up the working fluid, the flow rate will be low. If the heat capacity of the working fluid is too low, the heat energy that can be moved in a given time will be low.
Water hits a sweet spot of heat capacity and boiling point under pressure, and one which is hard to beat. There are lower temperature working fluids which get used for things like geothermal power plants using binary cycles or things using an organic Rankine cycle, because the underlying heat source isn't hot enough to generate enough steam to run a turbine.
This is why modern power generation still frequently use steam as the working fluid, centuries after the first practical steam engines. Look on the bright side though: hydroflurocarbonpunk just doesn't trip off the tongue. It'll never catch on.
(Note that I use a lot of weasel words above, because these things are always more complex than they initially seem, see also supercritical CO2 which can be an efficient working fluid at lower temperatures than steam, though it does require much higher pressures. That sort of thing isn't very steampunk though, so it doesn't fit your specific requirements so well)
edit: forgot to actually respond to your original question, oops
How could a fluid be better than water for steam power?
You working fluid should ideally be:
- minimally chemically reactive, even at high temperatures and pressures
- liquid at ambient temperature (so your engine doesn't congeal when it gets frosty outside)
To be better than water, it should probably have higher thermal conductivity in both the liquid and gas phase, but a lower heat capacity in the gas phase (so adding heat to the gas causes a greater pressure increase), though I'm not sure if there are any practical real-world materials that would fit all these requirements.
I don't think that a higher latent heat of evaporation is necessary or even desirable. It is useful for purely moving heat, but I think it makes it harder to develop pressure and for a steam engine that moves stuff you want plenty of pressure. If the thermal conductivity of the gas was higher, superheaters can work better which might let you lower the latent heat of evaportion and form more gas and high pressures for the same energy, but I'm speculating wildly at this point.