R. Yissocher Frand quotes two approaches here:
1. From R. Ya'akov Weinberg:
Ur Kasdim was something Avram did out of his own convictions; it was not a response to a Divine Command. Lech Lecha — leaving his home — was a Divine Command and hence the fact that Avram obeyed this Command was of greater significance than the fact that he was prepared to act under his own convictions. In the eyes of the Torah, obeying the Master of the Universe is really the greatest test.
2. From R. Simcha Zissel Broide:
[P]eople face two types of tests in their lifetimes. One type is those things that may be categorized as “glamorous tests” — defining moments in a person’s lifetime. At such times, when a person is well aware that he is at a junction in his lifetime when he must make a crucial decision — even one involving great self-sacrifice — he may be able to muster the courage and determination to make the proper decision.
However, there are other tests in life — the daily type of challenges that come our way, day in, day out — that each and every one of us face. These tests are not glamorous. They are not exciting. They are very run of the mill, but they are challenges nonetheless.
The test of “Ur Kasdim” was one of those glamorous defining-moment types of tests. The test of “Lech Lecha” is typical of the “daily grind” type of test. People sometimes have a capacity to rise above what they really are and to achieve spiritual levels above their normal capacities, but that is not who they really are ...
[I]n the eyes of the Torah, the day in, day out, tests, represented by “Lech Lecha,” are what really count. Everyone can rise to the occasion occasionally. However, to do what you are supposed to do every day — day in, and day out — without questioning, this is a real test.