It was surprising because we thought Kishi would make Obito use Rinne Tensei on everyone but instead, he lived up to our expectations and made Obito use Rinne Tensei on Madara.
Well, let me explain with a example: Suppose, you're gonna meet a celeb because of some special reason, and you know it will happen. But, when you actually get to meet the celeb, you feel super excited that it happened really!
Same is the case with this. We all expected Madara to force Obito to revive himself, but when it actually happened, we were at the edge of seats, as what we expected finally happened!