Because of the young male viewers TPTB tought were the main audience of Voyager.
Okay, kidding aside,
What kidding? You're right. It's the same answer as to why T'Pol wore a catsuit.
I don't dispute this, but I felt I should say something. I'm a heterosexual teenage male, and I roll my eyes whenever I hear people saying that we watch shows for women in catsuits. It strikes me as rather offensive, both to us and the women. There are far better reasons for watching Seven and T'Pol- they're fascinating characters portrayed by skilled actors. The whole young-men-are-sex-crazed idea really should be retired...![]()
The whole idea won't be retired for the simple reason that it's true. I was a sex crazed teenager twenty-five years ago, and now, at the end of my 30's, I'm still sex crazed. If you are a teenager, I'd imagine (and remember I speak from experience here) that it's even harder for you to keep your hormones under control. It mellows over time, but NEVER goes away completely. Trust me.

Don't take offense, it's a simple truth of biology.
Like it or not, you are the target audience both Seven of Nine and T'Pol were created for.