Final answer:
Wagon trains had to leave in late spring for three main reasons: adequate grazing for livestock, avoiding winter snows, and time to gather supplies.
Step-by-step explanation:
The primary reason wagon trains had to leave in late spring is because they needed to have adequate grazing for the livestock on the plains. Late spring ensured that the grass had grown enough to provide sufficient food for the oxen pulling the wagons.
Moreover, leaving in late spring allowed them to avoid the winter snows in the western mountains. This was important for the safety of the travelers and to prevent their progress from being hindered by harsh weather conditions.
Lastly, late spring was also chosen because it provided enough time for the immigrants to gather supplies, make necessary repairs to their wagons, and purchase any additional items they might need for the journey.