Is there a reason why female Mormon missionaries wear such varied dresses but male Mormon missionaries wear uniforms? Is there a reason why female Mormon missionaries wear such varied dresses but male Mormon missionaries wear uniforms?

I see male missionaries often. They are easily recognizable, because they all wear uniform outfits - white t-shirt, black pants, and a clean tie to finish. When I go on the Mormon site, the dress code for men seems more varied than what I previously thought, and the dress code for women is not only more varied but also more colorful and vibrant. 

My question is, is this true? Do Mormon missionary women get to wear more varied and colorful clothing than Mormon missionary men? And why do the men on the website wear more varied clothing with different-colored ties but the men on the street wear exactly the same style?
The missionary dress code has changed a lot over the years to stay compatible with current trends and styles (without compromising modesty, of course).
Until about 2010, sister missionaries were required to wear nylons and dark dresses/skirts that went down to at least mid-calf, as well as dark suit/jacket tops. In 2010, the dress code was updated, permitting:

Knee-length skirts instead of always down to mid-calf
Nylons not required
Bright, vivid colors

From this news report, an MTC employee said:

&quot;Having recently served a mission, I knew how people reacted to the floor-length skirts and generic collared shirts,&quot; Porter said. &quot;Sure, we looked professional, but not all that current and approachable. Being a part of the standards update ignited my excitement for missionary work. Now sisters could feel cute, comfortable and still modest. And that was the goal.
&quot;Everyone working on the project had the same collective vision,&quot; she continued. &quot;We wanted the sister missionaries to be more approachable, more up to date with fashion and as always, modest.&quot;

Basically, the change now allows sisters to look more natural, approachable and personable. It allows their personalities to shine through a little more, and that's something the LDS Church considers valuable when sharing the gospel.
Similarly, elders are now allowed to wear light-colored suits and slacks. Before 2011 or 2012, all their clothing (except the white shirts) were dark. To answer your last question about ties and suits, the lack of variation might just be because few men straight out of high school have good fashion sense. (I jest, but only a little.) Dark suits are still most common among elders, though, it seems.