We have seen that, upon arriving at the Wall, Bran and Meera are greeted by the men of the Night's Watch in S07E01, and Jon Snow left for the Dragon Stone toward the end of S07E02.
So was there any specific reason behind delaying this meet?
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The show, due to time-constraints, has progressively taken more and more liberties with how fast time goes by for the characters.
It seems to take no issue with skipping weeks or months when going from one episode to the next, for one character, but seemingly only days for another character. Euron Greyjoy being in King's Landing in one episode, then taking Dany's Greyjoy fleet out, then being back in King's Landing the next episode, only to be in Casterly Rock the next episode is one example.
This phenomenon seems to have cropped up more and more as the show stopped being able to base its writing on the source material (Song of Ice and Fire book series).
Bran Stark's arrival in Castle Black in S07E01, only to arrive in Winterfell in S07E03 seems reasonable by some standards. Don't forget that it took Jon Snow 3 episodes to travel from Winterfell to Castle Black in Season 1 (he arrives there in S01E04).
Jon Snow leaving and travelling half a continent by boat in half an episode is just an example of the aforementioned out-of-sync time compressions for each character.
There might be a plot reason, but it's too early to tell.
Bran mentioned to Sansa that he needs to speak to Jon about something. Given the revelations in season 6, this is likely to be about Jon's parentage.
For plot reasons, it might be important that Jon gets to meet Daenerys before he learns of his ancestry. Depending on what HBO is going for, they might wait for the two to become allies, rivals, lovers or enemies before the reveal throws a spanner into said relationship.