Why does biden not care about the poor elimination of college debt and making college free?

Biden cares about it all.

It's not that he doesn't care about it but that, one, there are bigger fish to fry right now and, two, he doesn't think it's financially feasible in the US's government's economic state. 

You have to remember that Biden is a moderate, even a centrist, which was the biggest criticism he got in the Democratic primaries and during the Democratic debates, and where he's most so is in fiscal policy. 

As much as the Trump campaign tried and the right-wing media continues to try to spin him as this alt-lefter, this left-wingnut radical, that's simply untrue. It's never been true. Nothing could be further from the truth. Biden is just left of center overall, but fiscally, Biden is as conservative a Democrat as there is, so much so that on fiscal policy, he is actually right of center. 

However, Biden does recognize that he's representing voters' interests, so with the strong leftward push on this issue, particularly so unanimously within his own party, he has moved towards a middle ground where he supports eliminating some college debt, just not all and not just now when there are more pressing issues that are already stretching the US government very fiscally thin.