This is the excuse that a leader will give you when it thinks that it has a tech that few other leaders have. Note that few depends on the leader: Mansa will always trade, but some others will trade when 30% or 60% (or even more, see Toku) of other leaders know the tech. Of leaders that this leader knows of obv.
How to persuade such a leader? Make him know more leaders knowing that tech or make him lose contacts with other leaders not knowing the tech