First, because it'd another huge expense for the country with no way to pay for it. Free stuff really isn't free, and we've booped on the booper valve way too much already.
But also, encouraging people to go to college sets people up for defeat unless they are prepared to go. College isn't some magic enlightenment machine or career-making thing that works independently of your effort. If you can't do college level work, it won't do you much good to go to college. And if you can't get a degree -- even an Associate degree -- completed within a reasonable amount of time, you're just wasting your time, delaying your entry into what level of the job market you can reach.
Our problem in the US isn't access -- anybody can get into community college. Nor is our problem cost -- anybody can afford to go, as long as one can reasonably expect to make the grades and finish. Our problem is too many young people are not ready to do college level work, nor do they have a realizable goal they can complete.
The President's proposal really doesn't benefit students, nor was it designed to. Its guarantee of funding would primarily benefit college employees (especially administrators), who are a reliably Democrat voting bloc. Which is the real point.