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):
Avraham had the ten trials, and Yitzchak allowed himself to be bound on the Altar; and yet, it is Yaakov who is the "select one of the Patriarchs," and whose face is engraved on the Throne of Glory. What is special about "יעקב איש תם יושב אוהלים," Yaakov, the wholesome one who dwelled in tents (Bereishit 25:27), that Avraham and Yitzchak, who withstood many trials, didn't have?
Rav Moshe answered that Yaakov dedicated himself "constantly and exclusively to Torah and mitzvos, which is greater than any trial.
It was this greatness possessed by Jacob, who abided in tents and devoted himself to Torah study, that merited him the titles and honors cited above.
The 10 tests of Avraham were exceedingly difficult.
perhaps it is because only Avraham had the potential to pass such difficult tests and be an example for humanity. it seems there was something unique about him.
some sources Job 38:2
"Who is this who gives dark counsel, with words, without knowledge" - Rashi: Dark and foolish counsel. Another explanation: Who is this who gives dark counsel… without knowledge? Said the Holy One, blessed be He: Who is this who darkens counsel, frustrates words, i.e., with a multiplicity of words; for I wrote: “and that man was sincere and upright,” at the beginning of the Book, in order that My name should rest on Him, but he came and darkened and frustrated what I counseled with his many words. Whereupon Job replies to Him (below 42:3): “Who is this Who hides counsel?” Had I known Your counsel, I would not have spoken so much. Then the Holy One blessed be He, answers him, “Neither did Abraham know; yet he passed ten” [tests].
Chovos Halevavos gate 10:
"Such too were the sentiments and conducts of the early pious ones who were subjected to trials, such as Daniel in the lion's den and his three companions in the fiery furnace, the ten martyrs, and those who conducted themselves as them"
Tov Halevanon commentary: those tests are not comparable to the test of Avraham, because the tests of other pious ones came from other human beings' wickedness and free will on them, and G-d helped those pious ones to stand up to the test, and sanctify His Name, sometimes G-d even performed miracles for them. But in the test of Avraham, however, G-d Himself tested him, and Avraham himself stood up to the test on his own, without help, out of his great piety.
"the greatest man among the anakim (giants)" Joshua 14:15 (Shemot Rabba 28:1, Vayikra Rabba 29:8, and elsewhere) explain the verse as referring to Avraham.