Why is water not safe to drink in mexico?

Because there are Mexicans there?

Water that drains off agricultural land often has animal waste in it. In the US, the water is heavily regulated and goes through an elaborate purifying process- that's why we pay for the water we use. Mexico can't afford that in all areas. However, I have to say that I lived in Mexico for a year, and never got sick. Then I came home to the US and got food poisoning at a party (most of the other guests had it too, so it was definitely from that party). People I know who live in the cities brush their teeth with tap water with no trouble, although most people have big water bottles delivered for drinking - like old time office water coolers. 

ALL tap water has some levels of bacteria in the water.

YOU become adjusted to YOUR local water and that bacteria doesn't bother you.

If you travel far enough from your local area, then you MIGHT be bothered by the bacteria that is in the water of the new location you go to.  The typical reaction is stomach discomfort and diarrhea.  It is temporary and goes away when your body adjusts to the water of your new location.  

Within your own country, the standards for purifying the water is usually similar enough that traveling within your own country is usually not an issue.  But when traveling to a foreign country, their standards may be slightly different and some bacteria levels in the water may be slightly higher than what you are used to.

And if you always drink bottled water and never drink tap water, then you should probably not drink tap water anyplace.  

Because it's contaminated with bacteria.

maybe cause of whats in the water