1) I only have to attack a few forts and one city while the Stormcloaks have to attack many cities.

2) I like the Legion's armor better. The Stormcloak armor doesn't look any different than a city guards armor. Thus destroys the Stormcloaks will not confuse me anymore.

3) The Jarl of Whiterun sides with the Legion.

4) I destroyed the Dark Brotherhood because I hate the Dark Brotherhood and the Imperials do too.

5) It fits very well with my story. I'm an Altmer and I join the Imperial Legion but I crush the Thalmor. Just like the story when the main bad guy's optimistic  assistant turns against him.

6) I covet Jarl Ulfric Stormcloaks clothes. And the only to get them is to join the Legion and defeat the rebels.