How could Troi promote to rank Commander so easily and why didn't La Forge, Data and others do that?

In the episode Thine Own Self Deanna Troi decides to participate the Bridge Officer's Test and gets promoted to the rank of a Commander after passing. Dr. Crusher did this before as we learn in that episode.

So, how is it possible to get the rank of a Commander so easily and why didn't other officers do this test long ago? Take for example La Forge and Data. Both are way more experienced in commanding, both have a lot more technical and scientific knowledge and I guess they could pass the test immediately without even training or learning.

Why don't all Lieutenant Commanders takes this test? If Troi is able to pass it, pretty much everyone else could too.