Is this supposed to provide more aggressive geometry, stiffer frame, or is it just purely aesthetic?

It looks cool, but it is starting to resemble the frame shape of beach cruisers...

lighter due to less material, and also stiffer.

there may be other reasons but they seem to be the main 2.

What newer frames? Pics pls? There are tons of variation so the claim that all "newer" frames have consolidated on some sort of sloping geometry is probably wrong. I could be wrong, but I don't see the trend you are proposing unless you post some pics.

I'm not sure how long it has been going on, but it seems especially prominent in specialized bikes.

Btw, I never said companies are converting to it, I'm just wondering why some have...

It may be lighter and stiffer, but I have a hard time believing that is true.
Is there any testing data that is "apples-to-apples" on that ?
I would speculate that the closer a triangle is to "equilateral" the stiffer it is. The main triangle and rear triangle of a traditional frame design look closer to equilateral to me.

I am just speculating, so if there is some real data, I'd like to see it.

Even though I am riding a sloped top tube frame I like the looks of classic geometry better. I hate the "swoopy" look of curved tubes. Don't even get me started on Pinarello !

In theory, it should allow people to be fitted to fewer frame sizes as well, thus less stock, fewer molds, etc. See here for marketing explanation. One of the main reasons though is that it's cheaper for the manufacturer - Molds are expensive! Setback seatposts, even size under the sun stems, and you can fit very different people on the same frame.

Sloping frame was a marketing idea first introduced by giant if I remember right. It was supposed that a sloping frame had the same geometry to a traditional frame but was "smaller" in order to maximize stifness and comfort while minimizing weight at the same time. The fact is that the sloping design helped the bike industry make fewer sizes (5 sizes were enough since you can adjust seatpost height and stem length) that's why everyone embraced it. After a few years "sloping" aesthetically equaled "modern" that is why it became the standard. A traditional frame usually looks wrong when its size its wrong. With a sloping frame you cannot really tell. IME sloping design alone does not make a frame better. Colnago insisted on traditional geometry for a while but marketing prevailed at the end. Anyway, there are many topics on this. Search...

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Wilier Cento Uno
Colnago Master X-light
Wilier 18carati

Kgt is absolutely right. I saw this initial post and thought about writing, but then thought, why bother... let people believe what they want. It is a business decision, pure and simple. The rest is marketing spin. With sloping frames you can fit more people with fewer sizes. Less frames that dealers have to stock and easier to sell. Colnagos (for example) which come sized in 1cm increments (for their traditional frames) are probably some of the most laborious frames to sell, simply because the customer will labor forever over getting the correct size since there are probably several sizes that will "work". And let's not even get into the time it takes to choose the correct paint scheme if there are several to choose from. At the beginning of the sloping framed road bike (Giant is the most notable), there were often only 3 sizes, Small, Medium and Large. Now most manufacturers realized that was too few and added more sizes. And some, Cervelo comes to mind, even though they have more sizes will still use the same length chain stays for all their sizes, which makes no sense to me at all except that it is obviously cheaper to produce with more common parts across the size range. Sloping frames have definitely become the mainstream in today's bike world. Some are nice, some are hideous, same as traditional frames.

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My Colnago C59
My Special Colnago EPQ

Sloping has no real effect on geometry, just determines how long you seatpost is. As others have stated, since there's a greater range of seatpost height, you can "fit" more people to the same frame, thus make fewer frames.

I once posted something about sloping frames being stiffer than horizontal frames on this forum and I got lambasted for even thinking that
(I do prefer sloping frames I think they look better)

Helps the MTB riders, a road bike that looks vaguely similar helps them to buy.....

Seems to me a sloping frame is not conducive to aero qualities - which appear to be a growing discipline in high performance road bikes.

I'm not sure, but my seat post is loooooooong. I have a big drop tho.

Sloping does not affect fit.

If you imagine a sloping frame to have a horizontal top tube, it does not change the geo one bit.
Why would it ? geo is not determined by the top tube being sloping or not.
Only thing might be it would look less off.

There has been a move towards more compact frames overall I think. Look at bikes prior to and up to into the late 80's and now. Fact is, if you can make a frame more compact, it will be stiffer while using less material, hence lower weight.
Then after that, manufacturers focused on oversized thin walled tubing to lower weight and increase stiffness.

Orginally, Giant pioneered this with the intention to produce lower weight frames I think - at that time weight was the 'marketing' challenge, now, not that much, loads of uberlight stuff around. The old tcr advanced were even lighter than those today. In those days, the pro's rode custom layups (not just giant, many brands) which were heavier than those you bought in the shop.

Yeah. A sloping top tube does not affect fit at all. The choice of manufacturers to make less sizes in frames does. So what? There are tons more bike companies now than 30 years ago. Just find one that fits. It ain't rocket science.