https://www.theguardian.com/world/2022/may/12/why-are-sweden-and-finland-not-yet-in-nato-and-does-the-alliance-want-them

The Guardian has a decent article on why Finland didn't immediately join NATO after WW2, and also explains Sweden too.

In a nutshell, Finland wanted to avoid possibly provoking the USSR into war. Joining NATO could be possibly interpreted as a threat, because NATO was effectively a force to push back the USSR's expansion during the Cold War. As such, Finland saw it prudent to go with a compromise. They didn't officially join NATO, losing out on some of the benefits of being in NATO (like protection in case of invasion), in exchange for being able to be friendly with NATO and the USSR at the same time. After all, it'd be hard for Finland to claim that they were at peace with the USSD if they were members of what was effectively a "fuck the USSR" club.

This arrangement used to make sense, because back then the USSR was somewhat weary of war. They'd need at least some tangible, somewhat justifiable reason to go to war. A country like Finland, which literally borders the USSR, joining NATO could easily be turned into a reason for war. By not joining NATO, Finland was demonstrating to the USSR that they weren't a threat.

The reason why Finland now wants to join NATO is because this arrangement no longer works. Putin has shown that he's willing to invade countries and start wars for next to no justifiable reason, as seen with Crimea and now Ukraine. The only thing between Finland and being invaded are mere whims. If they're going to get threatened, might as well get the protection.