Why was Avraham the only person that, according to tradition, was tested by God 10 times?
According to Kabbalah in Zohar, just like Avraham was tested with 10 trials, so is everyone. That means every person has 10 major tests in his lifetime.
Rav Moshe Feinstein in his Darash Moshe (Genesis 25:27, ArtScroll Judaica Classics, page 48):
explained that Yaakov Avinu did not have 10 tests because he learned Torah instead. As a result, Yaakov Avinu was the greatest and had the most honors. Rav Moshe explained that if you learn Torah you don't need the tests.