Lightest is easy, it's generally easier to travel with if you're not doing a serious dive, but want to be on a rebreather. That's why things like the GEM SCR exist.
As for popularity, I can see wanting a rebreather that has a wide user base simply because of the availability of parts, maintenance, broad familiarity, etc.
I don't necessarily subscribe to either of those being deciding factors in my own diving, but I can see how in an ideal world, a rebreather you can throw in your carry-on, and get parts for anywhere in the world, would be appealing.
While I'm under no false assumption that I could reliably get particular specific parts for either my SF2 or my Pelagian anywhere in the world, both of them are dead simple, and it would not be out of the realm of possibility to fabricate some parts in-situ if necessary. I could make counterlungs for the Pelagian out of motorcycle tires if necessary, and the bellows on an SF2 are a stock McMaster part. O-rings are generally available anywhere, and I could easily replicate a canister for either out of sewer pipe if necessary. Both use very commonly available 9v batteries, and really, outside of certain parts, all others should be easily fixable or sourced from something comparable with minimal fuss. Try doing any of that with an Inspo.
My Meg on the other hand, would be significantly harder to get up and running in the even of an issue. When it was a 2.5, if I lost electronics I'd be screwed. When I switched it to a 2.7 w/a fischer it would be less of an issue. However, short of the the neoprene lungs that you can fix with aquaseal, the stock lungs may or may not be repairable in the field, or at least not as easily. The ADV on my Meg was the plunger style, on my Pelagian and SF2 it's stock 2nd stage guts, much easier to fix than the Meg. Battery wise, you either travel with the 9v adapters, or you've gotta make a new battery pack out of AA's. Not a huge deal, but it could be a pain in the right circumstances, especially with more and more airlines getting twitchy with batteries, even though there's zero risk with alkaline's or NiMH rechargeables in a unit. Now, none of this is to say the Meg is a bad rebreather in any way, shape, or form. I'd still have it if I didn't get such a smokin' deal in trade for my SF2. They're damn near bombproof, and chances are you'd never have an issue in the first place, however, were something to happen in an austere environment, I'd rather have something I can fix or repair, than be forced to sit out.
So, that all being said, I can see the justifications for having a widely popular rebreather, and if it's lightweight for travel, having the popularity of the unit on the arrival end of travel would be beneficial.