What are you defining as the "old days" and who are you thinking of that could go under 1:02 in a half marathon but not 2:10 for the marathon? I ask because in what I'd call the old days the half marathon was rarely run and not done all that seriously when it was. I cannot think what guys like Rodgers, Kempanen, etc. did in half marathons so if you had some examples I'd be interested.
At any rate, I don't think there's just a single reason why 2:12 is sort of a settling place for lots of elite western marathon runners. One is that there's not a ton of incentive for westerners to go faster. In the US, 2:12 sets you up as a very legitimate contender for Olympic teams, shoe contracts, first American status at major races, etc. So does 2:10, 2:07, etc. but there's little additional "payoff" for attaining faster times. Yes, you might be able to win New York or Boston if you can get to the 2:08-2:09 level but it's a long shot and maybe not worth pushing yourself harder and risking injuries or blow ups. It's a very long way from even 2:10 to 2:05, it's uncertain you'll get there and economically you're not that much better off at 2:08 than you are at 2:12.
Another reason is that westerners generally don't run many marathons, certainly not as many marathons as half marathons. Naturally if you race a distance ten times you're more likely to turn in a stellar performance at it than you are at a distance you only run twice. Ron Hill's first marathon came in 1961 and took 2:24. His 2:09 came in 1970 and he ran at the very least two marathons, often more, a year in the intervening years. I have a friend who ran 2:16:54 in 1969 and 2:15:31 in 1983. In between those two races he ran over 90 marathons. It can take many tries to get your best marathon time and few westerners now take that many tries.
The typical elite US marathoner today runs one or two marathons to get a Trials qualifier, the Trials and if that goes well, and it usually doesn't, the Olympics. Then they lay low for a good while. I think if a lot of those guys ran 2-3 marathons a year for 5-10 years they'd work those 2:12s down to 2:10 or better. But they's also have to deal with a number of 2:15 plus races which would maybe convince them they'd started to slow and to give up serious marathoning.
In the US's "glory days" at the marathon the guys who were our best marathoners were also generally our best distance runners. George Young, Kenny Moore, Shorter, Bacheler, Rodgers, Galloway, Virgin, etc. ALL ran serious marathons while they were still among the best we had at track distances too. Our best distance runners today generally stay on the track. If they move to the marathon it's after they've peaked out at track distances. The guys in the marathon are simply not as good distance runners as the track guys are. If the best track guys ran marathons seriously you'd see sub 2:10 times pretty routinely.