Here is as good an explanation as I have seen. I was curious about this also when I was first looking at purchasing a bike last your for the first time in way too many years.

Well I can't post a link so here is a quote from roadbikerider dot com:

"The simplest answer is if the top tube is not constrained to being level with the ground, the frame’s designer is free to use any head tube length (which governs handlebar height), and any seat tube length (which governs standover height), she or he desires.

With a level top tube, as the frame’s head tube gets longer, the seat tube must follow suit, which often results in a battle between the ideal handlebar height and ideal standover height.

Sloping top tubes became more common on road bikes about 25 years ago, concurrent with the appearance of mass-produced mountain bikes, whose tall suspension forks and often unexpected rapid dismounts demanded a frame design out of the ordinary. For all riders but the very tallest, a level top tube mountain bike was not possible, or at least not safe, to ride.

As people got used to the unconventional look of a mountain bike, with its sloping tube, frame designers began to incorporate the sloping top tube into road bikes as well. Usually, the reason given was that the smaller front triangle created by using a sloping top tube resulted in a lighter, stiffer frame. All else being equal, this is true, as long as you never sit down when you ride.

By the time you add a longer, larger diameter seat post to the equation, though, you may find that you’re right where you started as far as weight and stiffness go.

Slope Skeptics
Slope skeptics also point out that many manufacturers limited the number of sizes offered with the new sloping frames. Rather than sizing frames in one- or two-centimeter increments from, say, 50cm to 60cm, frames were now only offered in sizes small, medium, large, and (maybe) extra large.

The reason given by the manufacturers was that by sloping the top tube, and therefore taking standover clearance off the table as a fit element, it was now possible to fit more riders on fewer sized frames. (It did not go unnoticed by the skeptics that it also greatly reduces the inventory burden of the manufacturer.)

In the end, some people see sloping top tubes as an affront to the elegant look of the traditional road bike, a cost-saving measure disguised as an evolutionary advance. Others like the look, and appreciate the flexibility in design the slope allows.

Personally, I see sloping top tubes as another tool in my design toolbox. I’ll use a gradual slope to pick up a little extra stand-over clearance, and I’ll use a steeper slope if the customer requests it. Similarly, I find that many riders appreciate the classic look of a level top tube, and if that trips your trigger, I’m happy to build a bike that way, too!"