Because what is polar and more hydrocarbons are non-polar. The rule rule "like dissolves in like."

Hydroxyl (-OH) groups are very polar. But alkyl (-CH3, -CH2CH3 etc.) groups are non-polar. So the more alkyl groups (carbons) a molecule has the less water soluble it will be, while the more polar groups like -OH it has, the more soluble in water it will be.

You can see this trend in action. Methanol (CH3OH) and ethanol (CH3CH2OH) are both water soluble. But 1-hexanol (C6H14O) is only partially soluble in water. Compounds that are less polar, like diethyl ether (C4H10O) aren't soluble at all in water. When two compounds are mutually insoluble they're immiscible with each other: water is not miscible with diethyl ether.

So when you mix hydrocarbons with water, they tend to form layers. Water is more dense than more hydrocarbons, so it is usually the bottom layer. The organic (hydrocarbon) layer is usually on top. Some common exceptions to this rule are chlorinated hydrocarbons, such as chloroform (CHCl3) and dichloromethane (DCM, or methylene chloride, CH2Cl2). Both chloroform and DCM are more dense than water, and so would be on the bottom later if placed in a sep funnel with water.