It doesn't really say "why". Water ionizes really conveniently so you can make it an acid OR a base (which is sorta unusual and it can talk to acidic or basic things), and is polar (its got a + and a - electrical side, more or less, so it can talk to + or - things), and has hydrogen bonds (so you get liquid lifestyle at gas temps)
For one why, look up amphoteric and zwitterion. Water has a super convenient pH range where about a ten millionth of pure water self ionizes aka ten to neg seventh or pH of 7, and all kinds of super convenient reactions occur above and below that pH and its really easy to manipulate ionization rates around that level without using too much. You don't need just a liquid but one where you can really screw around with something like amino acids by easily and cheaply changing the ionization rate of the liquid. So there are super convenient chemical reactions that depend on the ionization level of the liquid and its really easy to manipulate water. Conveniently water "just works" without having to add tons of other stuff to it.
Liquid methane isn't polar enough to really be useful when messing with ionic substances (table salt, etc). Whatever you use for a liquid, it needs to be polar so ionic stuff like salts can dissolve. It turns out that interesting chemistry doesn't happen with non-polar substances at normal temps; thats why when you bury stuff for a couple million years the only thing left behind undecayed (more or less) is non-polar hydrocarbons (crude oil). Ammonia is polar but has other issues.
The hydrogen bonds are important. Water should be a gas at room temp. Really, it should, looking at bonds and molecular weight and stuff. Yet the hydrogen bonds that form keep it liquid at room temp. So you get "gas speed" chemical reactions at "high" room temperature, yet its liquid to a ridiculously high temp.
Maybe some custom liquid silicone with some bolted on weirdness could make a useful artificial blood plasma or "stuff" for life to live in. How it would make it without the chemical plant being made first is mysterious.
Sometimes I wonder that not only it is not possible for other forms of life to exist that wouldn't use water, carbon and others, but that we have life just because it so happened coincidentally that our universe were created with the exact properties needed for the laws of physics to support life for a short while in some places.
Those are non-polar, so would not work as a liquid environment for Earth-like biochemistry. Doesn't rule out the possibility of different forms of life ... but there is a high prior against it.
I'd order the candidates for the basis of lifelike chemistry complexes as follows: H2O, NH3, SO2, HCOOH, CH4.
A lot of the common molecules aren't good solvents or react too readily with other common elements or molecules, like oxygen or halogens. And molecules that are good solvents just aren't common enough to dissolve much of anything.
Methane would probably only work in an oxygen-deficient environment, and ethane wouldn't form enough by coincidence to be a meaningful solvent in comparison to methane or other simple organics.
And it just so happens that H2O is very common and a great solvent, so any other potential solvent would probably have a little water in it anyway. And if it's there, a lifelike chemistry complex would probably make use of it in some way.
