Because if I joined you, your people would inevitably do what my people did in the war.

A general word of experience — nobody brings their attack ships without full-well meaning to attack. They already know your word against the gathered intel and suspicion on the ship wouldn't amount to much. They merely hoped that they could spare some of their battle ships, should you be able to actually convince us.

That is one thing I can credit your new people for — they are careful with their resources.