There are a number of reasons the "triples" have fallen out of favor.
- With the new cassettes, the freewheels have shrunken, so for vintage bikes, 13-x or 14-x freewheels have been replaced with 11-x cassettes.
This change causes a couple of things to happen. A 11/33 cassette would have the same 3:1 reduction as a 14/42 freewheel 
This, in turn has allowed the use of smaller cranksets. So, say a 50/34 is a very common size today while the vintage bikes might have a 52/42 crankset.
And the 50/34 crankset competes favorably with the 50/40/30 triple.
- More sprockets on the rear. So, a vintage 10-speed would have 2x5 gearing, or for an 18-speed, 3x6 gearing.
Now one might have 2x11 gearing or 22 speeds with a double.
This allows tighter steps between rear sprockets, but also wider range cassettes (coupled with longer rear derailleurs).
- Keep in mind that it is all about gear reduction. So, say a 50/40 front gives one about a 20% gearing reduction. An 11/12 on the rear gives about a 10% reduction in gearing. Skip two (11/13), and one gets close to the same 20% gearing reduction as the front.
Thus, add a couple extra sprockets, and one can get essentially the same gearing as one had with the triple.
Say one's vintage bike had: 50/40/30 up front, and 11/30 in the rear.
One's new bike has 50/34 up front, and 11/34 in the rear. The overall gear ratios are IDENTICAL. Toss on an 11/42 in the rear, and one now has lower gearing with the double than one had with the triple.
Front shifting can be a bit of a pain, and there is a lot of gearing overlap. That is one reason why SRAM (and others) are pushing a single front chainring (1x) with 11 or 12 rear sprockets.
It is best if the rear derailleur is long enough to wrap enough chain for the big/big and small/small sprocket combinations. And, one doesn't really gain much by having the extra front sprockets.
I still like triples for loaded touring and hauling cargo (I think). At least they are worth experimenting with. But, the double is fine for most ordinary riding including typical hill climbing. Probably for those wild days of lots of 20% climbs, I'd be better off simply installing a larger cassette.