Jesus told the Jewish religious leaders that He
was willing to answer their question on one
condition. That was that they answer His first.

"The baptism of John, whence was it? from
heaven, or of men?" Matthew 21:25

These men began whispering among them
selves, while the people watched. They were
embarrassed, John had testified of Jesus and
proclaimed Him as the Messiah.

Jesus had a right to their answer before He
gave them His. If they said that John was a
true prophet, then Jesus would ask why they
didn't believe Him. But if they said his baptism
was of men, then they would be shown up in
front of the people because they believed that
John was a prophet.

The Jewish leaders were in a spot where they
felt they didn't dare to answer Him. They were
forced to reply "we cannot say".