Why so few fluid phases?
The solid (crystalline) state is defined by structure, for which there can be many. Liquid means loss of long range order, unless you talk about liquid crystalline, where orientational, but not positional long range order, is partially retained. In the gas phase, also short range order is lost.

So, in the liquid or gaseous state, phases can only differ by the molecular structure of the constituents. For water, that leaves only one stable possibility. Sulfur has a few different molecular modifications, but they're hard to separate and they tend to interconvert, so they're not really phases.

There are a few liquids with two phases which differ not in structure but in quantum states. Helium I and II are an example, para and ortho hydrogen another.