During the whole season 7,
Jon Snow prepare the North to fight the Great War, and interact with Daenerys and Cersei.
But why did he never send raven to all major and minor lords of seven kingdoms to ask for help ?
Because that's a lot of ravens. More than Winterfell possibly has available. (and why would they have ravens that are primed to some minor castle in, say, the Reach?)
Instead, he sends a raven to the central information hub, the Citadel of Oldtown, in the hope that they will distribute the message to all maesters, and thus all lords, in the realm. They also have much more authority in this question then he has. To the southern lords, he is just another rebel king in the North and they probably would not believe him about such an outrageous claim.
Additionally, he tries to convince the two queens, which would have much more effect than him asking the minor lords personally as well. The lords are bound by their oaths to fight for their respective queen, so they can't just abandon them in the middle of that war to fight in the North, or they'd be traitors - unless the queen gives that order herself.
Feudal systems are a hierarchy. The minor lords are beholden to the Lords Paramount of the seven kingdoms who in turn are subject to the king (or queens in this case). They can't really go on their own. At most he'd pick up a few dribs and drabs that way.
Convincing a Lord Paramount would be better. But he already has three of the kingdoms: the North; the Riverlands (Sansa, Bran, and Arya are Tullys through their mother); the Vale (Littlefinger brought them to save Sansa). The Tyrells and the Martells pledged to Daenerys, so there's the Reach and Dorne. All that's left are the Crownlands and the Westerlands, where Cersei is the Lord (Lady) Paramount.
Another problem is that if a minor lord or a Lord Paramount would ride north to help Jon, Cersei would assume that they would be organizing against her. So she'd sack their castle on principle. In season 7, she already did that to
the Tyrells. And Dorne is likely only safe because it is so far away.
Convincing the two queens means that everyone can safely (absent treachery) send forces.
And frankly, Jon Snow's problem is less about getting enough of an army and more about getting enough dragonglass. He needs support from Daenerys for that. The Unsullied and the Dothraki are enough of an army for anyone. The Unsullied are nearly perfect if your main concern is keeping your casualties from joining your zombie enemy. They are actually disciplined enough to follow protocols about leaving no unburned bodies behind, even if they have to leap into a fire to do it.
Finally, who would believe him? If someone wrote you and asked for help preventing the zombie apocalypse, would you believe them? Or would you mark that message as spam and forget about it? Ned Stark had lived in the North all his life, and he disregarded reports of white walkers as myth. Cersei found it laughable...until she saw one. The typical lord would probably assume that it was something involving the three monarchs rather than a serious plea for help against the zombie horde.
Convincing the queens gives him help, a safe (sort of) space to prepare, and is the most effective way to get help fast. He only has to convince two people.
There is no need to send Ravens to all the Lords. Because, we could see most of the Lords are sworn to either Cersei or The King in the North (Jon himself). A meet has been arranged with the Queen of Seven Kingdoms. As a result, it is assumed that all the Lords are expected to serve for whom they are bound to.