Cuba is hoping to replace Venezuelan oil with American tourists

Between 1991 ish and now the cold war ended, but nothing much really changed since then. It was like the entire world was just letting the status quo sit for a while so see how things settled after such a large political change (e.g. would Russia turn into a western-style democracy?).

And I think the Ukraine incident gave everyone (mostly europe and the US) a wake up call that not only has Russia not changed, but it is likely not going to change. So now you have Russia buddying up to Cuba again, and you also have a Russia which is a legitimate threat to world peace (which isn't to say I think Russia will attack the US, I mean world peace in the sense of Russia picking fights with its direct neighbours).

So the US had a choice to make, they could either keep ignoring Cuba while keeping its evidently ineffective embargo going, or they could try to buddy up with Cuba. Given Russia's current financial position (read: bad) and the US's position (read: good) it makes a lot of sense for Cuba to pick the US right now over Russia.

The money forgiveness is just the "cherry on top." The fact that Cuba is now largely out of Russia's pocket financially just makes it even easier for the US to influence Cuba, which they've already started doing.

But ultimately with or without the forgiveness Cuba needs trade and tourism from the US. Russia cannot offer that logistically. I'd describe it with this analogy: "Russia and the US are both employers, but you owe Russia money, Russia says that if you work for them they'll reduce your dept but pay you nothing, the US says that if you work for them they'll just pay you normally and if you wish to pay off the Russia debt so be it (but can keep it after)."