According to Osho, Krishna praises the Gita right then because he senses Arjuna is on the brink of Krishna-like clarity and will soon speak to others. Krishna prepares him for transmission: instructing discernment—share only with the willing, devoted, and disciplined—and inspiring courage by extolling its blessings. This averts two calamities: spreading it to the unprepared or withholding it out of fear.
Krishna saw Arjuna was almost ready to teach, so he told him how and whom to tell—and praised the Gita—to stop him from talking to the wrong people or staying silent.