Because you usually heat up more of it.
Let's say you wanted to make a product that required heated alcohol. Alcohol costs money and hot alcohol is dangerous in numerous ways, so you'll only heat as much as you need--if you need 20cc of it, you'll heat 20cc of it. But water, that's different: you'll heat a whole quart and make tea out of the unused quantity.
Water requires less heat to change its temperature compared to most other common liquids. This is due to its high specific heat capacity, which means it can absorb or release a significant amount of heat with only a small change in temperature.
There are many liquids more dense than water. The most commonly thought of is probably Mercury.
Microwaves heat only 3 things. Liquids, Fat, and Sugar. Liquids and Fat are the most easily heated. If you are trying to heat oatmeal with no water or anything, it will not heat up because it does not contain much Fat or Water so it will not heat up. Just like trying to heat up a napkin.
To heat most liquids.
Water is different from other liquids because, unlike any other liquid in the world, water can exist in all three states of matter; water vapor, water, and Ice.Water is different from most substances in that when it freezes, it takes up more volume than when it is a liquid.
Since most of the time we are concerned with heat being transferred via conduction, the denser the material, the easier it is to conduct heat. Except for the rare anomaly (think ice vs liquid water) solids are denser than their corresponding liquid forms. All that is a gross simplification of course. Many liquids heat quite a bit better than solids and convection (which can occur in liquids but not solids) can greatly aid in the speed of "heating up", so the generalization that solids heat up faster than liquids is only a tendency rather than a rule.
Electricity holds the most heats
Yes, water is one of the exceptions to the rule that a solid shrinks when it solidifies. Water expands. If you freeze water in glass eg beer bottles, the bottle will shatter as the liquid freezes. Water pipes can burst, metal water bottles will split if frozen full of water. The water takes in air as it freezes, thus ice floats in your glass rather than sinking.
The solubility increases as the temperature increases. This is due to the fact that there is more energy available the higher the heat.
Water is the most common liquid on Earth. Other common liquids include milk, juice, soda, and oil.
There are quite a few liquids that are susceptible to bacterial growth. Water and sugary warm liquids are the most susceptible.
Many solids will change into liquids if you heat them, for instance most metals will melt when subjected to heat, ice will become water when heated, some plastics will melt, glass will melt, and most rocks will also melt.