Basically the rules are, you can't be in a fight already if you want to join.

Nato is a defensive organization. If any member is attacked, every other member has to come to their aid. You can't join if you're currently being attacked. That's a pretty fundamental assumption. Nato exists to prevent future wars by acting as a bloc, not to force their members to join existing wars.

To join nato, ukraine needs to win the war and recover all their territory, release their claims on any russian occupied territories, or get enough of nato's membership on their side to be agree to be forced into the war. None of these are possible anytime soon.

In the end people don't want to sent their sons to the meatgrinder if not absolutely necessary. There are the obvious exceptions of the rule.

Article 10 of Nato

The Parties may, by unanimous agreement, invite any other European State in a position to further the principles of this Treaty and to contribute to the security of the North Atlantic area to accede to this Treaty. Any State so invited may become a Party to the Treaty by depositing its instrument of accession with the Government of the United States of America. The Government of the United States of America will inform each of the Parties of the deposit of each such instrument of accession

NATO is a group where everyone agreed if one of them gets punched, they all punch the bully back. Because of this, the bully doesn't punch anyone in the group. The bully could make the punches painful for NATO, but NATO could do the same to the bully.

Ukraine is being punched. If they join NATO, all the NATO states have to punch back, and then the bully punches NATO because they're already being punched.

The threat of being punched keeps NATO and the bully away from each other. If one side actually starts punching, the other side would too, and both would be punched. So NATO isn't willing to let Ukraine in and immediately start a fight

Russia has a bomb strapped on itself, saying "If any one tries to beat ME up, I'll blow us all up!"

Today it would be about the best for me - I'm old enough that I wouldn't be drafted, my kids young enough that this would likely be over before they are old enough. However I have nieces, nephews, and cousins of military age, some currently serving - all of them are in danger of getting killed if we were to let Ukraine join and thus I cannot be for it. I have a lot of sympathy for Ukraine, but not so much that I want my close friends and family to die for them.

The above or some variation represents most of the people (not countries) who are in NATO.