I would say American lobbying and propaganda in Sweden and Finland, which we've had for decades. We were already de facto members anyway though. On the other hand, it's not like we had a choice. We voted for anti-NATO parties who, after the war started, changed their position and rejected any referendum and refused to wait until the autumn elections. Polls showed that a majority in Sweden didn't want to join if Finland didn't (and a majority did if Finland did), so you can effectively restrict your question to Finland (maybe someone else here has more details on their case.) There was a good reason for joining if Finland did though. We dismantled a lot of our military at the end the cold war and so we relied on an exclusive partnership with Finland; thus, without them, we'd be pretty weak.
Losing some amount of autonomy to the U.S. is far preferable to what we are seeing in Ukraine, or even Belarus.
Edit: That being said, Sweden and Finland were already quite integrated as NATO partners. It's unclear to me, a layman, how much is really changing vs. just being formally declared.
Not just to the US, but to Turkey. From day one, they demanded we abandon the Kurds abroad, brand them all terrorists and hand over political refugees to them and, starting today, sell them weapons. We had a ban on weapons export to Turkey but now half of our biggest parties refuse to comment on whether that will be upheld.
> what made Sweden and Finland finally want to join now after all these years?
Honestly I suspect that's just a momentary bout of mass irrationality (to put it politely). Both countries are completely safe, de-facto NATO members. Russia has no stated or implied interest in invading either. More importantly, Russia has no capacity to do so - they can barely push 50 km into Ukraine as it is.
America, otoh, has a lot to gain in extra weapons sales (remember NATO comes with a 2% of GDP spending target).
> America, otoh, has a lot to gain in extra weapons sales (remember NATO comes with a 2% of GDP spending target).
Maybe, but not too likely. OTOH, if you have numbers I'd like to see them. We make our own very high-tech weapons, which have already been NATO standard forever. We even sell some to the US (and Ukraine) and train them. We have been de facto NATO partners for a while.