Why does the hydrogen in the core of the sun not get so hot that it ignites?

It depends on the sense in which you mean it ignites. The sun already ignited nuclear fusion of hydrogen billions of years ago, a reaction which has enough hydrogen fuel to continue for another several billion years. The sun cannot ignite the combustion of hydrogen as there is not enough oxygen to support combustion. At 27 million degrees Fahrenheit, the core of the sun is actually too hot for molecules such as water to exist anyway.