Why didn't you journey with me? A literal translation, but rejected.....why
"To journey" is a pretty rare word, so it's likely not in the system yet.
because "viajaste" is past tense while your sentence implies sthg that is yet to happen.
Why do you want to put one there? "Personal a"? That's only for direct objects.
In general, conmigo is a preposition-pronoun combination, so the object you're talking about here already has a defined role in the sentence. Putting two prepositions (a and con) right after each other is a no-go in Spanish.