Although everyone pretends to know exactly why people, and women, in particular, are attracted to taller partners, no one knows for sure. There are several theories on why a taller partner is preferred over a short partner.
The Evolutionary Theory
A popular theory is that women like taller men because it shows physical prowess, and from an evolutionary standpoint, a more physically fit man was able to offer more benefits. For example, a more physically healthy man was able to protect women better, and their offspring were seen as more capable of having children and were better able to provide for women.
We're not saying this is a conscious decision women are making—quite the contrary. Women are conditioned by thousands of years of biology to favor taller men because of the benefits they could provide from an evolutionary standpoint in the past. These needs and considerations no longer have a place in modern society, but they were hardwired thousands of years ago and take time to get rid of.
The Ratio and Conditioning Factor
When considering what is attractive and why, factors like conditioning are essential; traditionally, women have been conditioned and taught to believe that being smaller is better. Women have been taught that being above a specific size is undesirable and have concocted millions of ways (healthy and not) to try to reach those unattainable goals.
One argument for why taller partners are preferred is the illusion it creates. If your partner is considerably taller than you, it gives the illusion of being smaller and more delicate than you really are. So a woman who is seen as larger than what society's standards dictates are acceptable seems acceptably small and light beside a much taller partner.
The Protection Mandate
A common reason people claim to like a taller partner is that it makes them feel safe. Having someone who is physically larger than them makes them feel physically cared for and protected. This is undoubtedly a hold-over from evolutionary conditioning that leads to a desire to be protected from danger. This desire to be protected isn't necessarily a bad thing; the desire to feel safe is a basic human need.
Is Taller Better?
Ultimately the question of why height is more attractive comes down to personal preference. For example, one person may have no idea why they like a taller partner, and the next might know they like a taller partner because it makes them feel safe or because they look good together.