I'm by no means a specialist, but I think that falcata and similar weapons demanded better and more extensive training than Roman gladius. The main question is - whether falcata was used by the "elite" units or was it a common weapon in Thracian armies? If it was an elite weapon than the answer is simple.

The Roman strategy was based on simplification and uniformity of the army. As opposed to many other ancient nations the Roman legion was a relatively uniform unit. Most soldiers had similar equipment and similar training. That was one of the most important inventions of the Romans. The army was no more a mix of few "champions" or elite units and a large mass of militia with poor equipment and even worse training. When Roman general was leading his men he knew what he can expect from them, what they can and can not do. The result of this uniform army structure was the unification of the weaponry and training, so the choice was made in favor of cheaper, easier to use, but effective gladius and not an expensive difficult to handle falcata.