Why only one ring at the front these days -- why didn't they used to be like that?
I suspect the main reason for this is the availability of narrow-wide chainrings, the invention of derailleur with a clutch, and the increase of sprocket counts in the rear.
Previously, there was no way you could achieve enough range of gears with only one chainring because to achieve reasonable steps between gears, you'd have maybe some 11-34 tooth cassette that only has little over 3x range. Also you'd need some guide to prevent the chain from dropping. If the guide is perfectly successful in its job (never rubs on the chain, prevents 100% of chain drops) then a static guide is enough but even some small percentage of chain drops not prevented means you have to have some mechanism to move the guide without getting your fingers dirty in chain oil, and hey presto, the guide became a front derailleur.
Also complicating the fact is that 11-tooth sprockets have horribly short wear life and are massively inefficient. The same is true but to a much smaller extent on 13-tooth sprockets. So to have an acceptable wear life and efficiency on your flatland gear, it means your flatland gear should be the 15-tooth sprocket at a minimum (or perhaps even a 17-tooth sprocket). If the big sprocket is 34-tooth, the lowest gear is only 2.27x easier than the flatland gear. It may be a limiting factor should you encounter a very steep hill.
Today, narrow-wide chainrings and clutch derailleurs are available. They together reduce the risk of dropping the chain from the chainring to a very small value even if you don't have a chain guide.
Also with 10-sprocket (or more) cassettes, it became reasonable to have something like 11-42 tooth cassette. This has 3.82x gear range that ought to be enough, and the jumps with gears are suitable so you won't find that you are missing a needed gear in the middle.
There are still three advantage of multiple chainring setups for users of trigger shifters. Those shifters allow quick downshifting over multiple gears but don't allow quick upshifting over multiple gears. So if encountering a steep hill, you can quickly find the gear needed for that hill. However, after the hill it's a different matter. If your flatland gear is 15-tooth sprocket and you used the lowest gear for the hill, it usually means with a 12-speed system that you need to click 9 times the trigger to find your flatland gear. With a multiple chainring setup, fewer rear sprockets and front derailleur, you find the flatland gear faster.
Also theoretically it's possible to damage your chainring by accidental contact with a rock. On a multi chainring setup, you might find the smallest chainring still works, and if it doesn't due to the rock-damaged big ring obstructing it, a very quick bend with an adjustable wrench in your emergency toolkit bends the obstruction into the opposite direction. You don't even need to fully repair the damaged ring on roadside, only a very quick bend will clear the obstruction. With only one chainring damaged by a rock, your only option is to try to repair it fully.
Furthermore, with a multiple chainring setup, and small sprocket count, it slightly reduces a crooked chainline if used properly. This means the extreme gears can actually be quite efficient. However, on a single chainring setup, the extreme gears have a crooked chainline that reduces their efficiency. However, of course this requires too some knowledge by the rider of the bike that a good chainline should be maintained on a multiple chainring bike. Actually with a multiple chainring system, if cross chaining, the chainline can be even worse than on a 1x system.
Today the 1x systems are prevalent on hybrid, mountain and gravel bikes but not yet on road bikes. The main reason for that is probably that on a road bike you can't realistically have smaller than 50-tooth flatland ring (otherwise the rear sprocket for your flatland gear would have terrible wear life and efficiency), so a 1x-system would need huge big sprockets in the cassette to have low enough gears, increasing weight. It probably adds less weight if the low gears are implemented by a second small chainring. Also when riding on the road, you can usually anticipate your near future gear needs a long time in advance and have time to shift to the proper range (selected by the front derailleur) when riding on flatland. For mountain bikers, it's harder to anticipate gear range needs so much in advance. It makes more sense to have only one shifting system, one range, that allow executing every shift even under load or with minimal interruption in load.