The Noam Elimelech explains that miracles that HaShem performs don't sway people to teshuvah. Rather miracles that HaShem performs on behalf of Tzaddikim bring people to teshuvah. Rashi brings two things that Yitro heard, the splitting of the sea, and the war with Amalek. Both of those two miracles were unique in that they were brought about through an "awakening from below." By the sea, it was only because of the merit of Yosef that it split, and by the war with Amalek it was characterized as a battle for the sake of HaShem--Bnei Yisrael fought in HaShem's name.
So, we can learn from Rashi that it is specifically the miracles that HaShem works out of his love for the Tzaddikim that inspires people to teshuvah.