As a female with the 'perfect' hourglass shape, these are some opinions I have for why this shape is not perfect. I also looked online for further information with the conundrum.
Most jeans conform to a regular shaped female. Therefore, if the jeans go over the butt, they are not going to fit the thin waist. They often fall down without a belt, and wearing the belt makes the butt look bigger and unproportioned.
If you have a big butt its going to extend to your thighs. Not a big deal-if you're tall. I'm a short girl, and my big thighs are unflattering for my height. My constant issue is trying to slim those thighs down.
It's not the most sought out anymore
Guys love curves. Sure they do. However, with the weight boom the majority of the world is facing now, men tend to gravitate to a more rare view. They're more forgiving of fewer curves for the natural skinny that most girls envy. This isn't my opinion either--
This website suggests the hourglass figure is 'a bit too big'
Can't handle stress
The same website says women with hourglass figures cannot handle stress. They say that Cortisol (a hormone in the body that deals with stress) puts weight on the waist which makes the stress easier to handle. It also leads to a bunch of health benefits that hourglass girls miss.
The hourglass figure is hard to maintain. Keeping the weight away from the waist just naturally happens, but it still affects the stomach, especially when you're stressed. At least the hourglass holds weight well, but a small waist with a big stomach makes you look pregnant!