Why does cracking turn a mixture of liquids from crude oil into mixture of gases?

That's not QUITE what it does, or at least it's not ALL of what it does.

For similar compounds, in general, lighter molecules boil - that is, become a gas - at a lower temperature than heavier ones.

"Cracking" is the process of splitting large, heavy molecules into smaller, lighter ones. Because they are lighter, they boil at a lower temperature, and so will become gases when the heavier starting material would still be a liquid.
It takes the long chains of unneeded hydrocarbons and splits them up into smaller hydrocarbons, the shorter the chain, the lower the boiling point, this is why Methane, Ethane, Propane and Butane are all gases at room temperature. Pentane however is liquid at room temperature as it's boiling temp is higher than room temp.