I *THINK* it's because the tail of front derailleurs fouls VERY wide tyres*. I'm not a fat tyre user so it hasn't been a problem for me ...
Perhaps long chain-stays also solve the problem? I don't remember it BITD of early MTBs - which all had triples <\sweeping statement> .
*personally I wouldn't choose such BIG tyres on a "touring" bike (and it's not an issue in CX racing), but that's getting into a whooooooooooooooole can of worms ... :P
This, I have a Marin Gestalt 2 Gravel bike that Marin says will take maximum 700x35 as the front derailleur arm is very close to the tyre, it's currently on Fulcrum Racing 800 Disc Wheels fitted with Vittoria Terrano Dry 700x33, which actually inflate to something like 35.2mm on the rear rim, the front derailleur arm is very close to the tyre, I believe fitting a 650b wheelset would allow a 47mm wide tyre to be used.
As for touring use 1x forget it, too many steps in the gears, perhaps something with GRX 810 groupset with a double 48/31 up front and 11-32 on the rear, also bear in mind luggage fitting, do they want panniers, or going bike packing style with frame bags? I think saying I want a gravel bike can be a real head scratcher as some are like strong road bikes, and at the other extreme are drop bar MTB's, the Marin Gestalt 2 like mine has mudguard fittings, would take a rack & Panniers, but is supplied with a Tiagra Groupset 50-34 and 11-32 cassette, and is nearer the road end of things, perhaps a smaller crankset would suit, and the drops are flared but not excessively