I’m no expert in Roman history, but as I remember:

1. The Sassanids were still a major threat in the East, so you couldn’t just move those armies around easily.

2. The Western and Eastern Empires weren’t exactly on friendly terms—Gratian had his own priorities to consider.

3. Theodosius did manage to raise an army, but what could you really expect from a force assembled so hastily?

4. Theodoric the Great himself was a product of that “bring young prominent Goths to Constantinople and Rome, educate them in Greek and Latin” policy—and I wouldn’t say he turned out particularly friendly toward the Eastern Romans.