Why is water flavorless?
Consumer water is not flavorless. Sources of flavor include (1) the chemical and microbial content, which is most influenced by geology and ecology; (2) chemicals added or removed during water treatment, and (3) inputs and reactions that occur during distribution and storage (Dietrich, 2006).

Two examples of ecology-related flavors are geosmin and 2-methylisoborneol, which are respectively responsible for earthy and musty odors, produced by cyanobacteria and actinomycetes (Izaguirre et al, 1982). Humans detect these earthy and musty odors at concentrations of only a few ng/l. The ability to smell geosmin and 2-methylisoborneol is affected by other factors, such as the presence of chlorine, which masks odors (Dietrich, 2006). 

The capability of organisms to detect microorganisms like cyanobacteria is not trivial, as cyanobacteria produce toxic microcystins with hepatotoxic and neurotoxic properties that can cause illness and even death (Blaha et al, 2009).   

In my opinion, from an evolutionary perspective, it would be more important for survival to taste pollutants in the water than to mask these components by generating a taste to water that could potentially make the detection of toxins less effective. 

References
- Blaha et al., Interdiscip Toxicol (2009); 2(2): 36–41
- Dietrich, J Water and Health (2006); 4(Suppl 1): 11-6
- Izaguirre et al., Appl Environ Microbiol (1982); 43(3): 708–14