Here's an article about Milton Caniff's work and an upcoming exhibit thereof. The piece seems to suggest that the Steve Canyon strip lost popularity near the end because it was a military strip and the nation was turning against the Vietnam War. I don't think I'd draw that connection so directly or fervently. All adventure strips, including those that in no way reflected the war, were losing popularity then at about the same rate. In addition, Steve Canyon was in an awful lot of newspapers that, through no fault of Caniff's, went out of business.
And there was a third reason that I suspect mattered more than Caniff's politics. It's that his strip, the last decade or so, wasn't all that wonderful. Al Capp had the same problem with Li'l Abner. It wasn't a very good strip near the end and while Capp, perhaps understandably, preferred to blame other factors…well, just read those last few years some time and see if he wasn't coasting on his rep. (The shrinking size of all strips was yet another problem.)
Caniff had simply burned out on his own strip by his last decade. Dick Rockwell was doing most of the art and he was getting old and not doing great work, either. Milton didn't seem to know what to do with Steve Canyon, as witness the many silly "Steve is dreaming" storylines that imagined the character in other time periods. To the extent Vietnam damaged the strip, it was because Caniff didn't know what to do about that war in his continuity and couldn't come up with an interesting alternative. He was a brilliant man and a brilliant artist, and when he was at his best, he was better than just about anybody. But his best wasn't those last ten years. It was all of Terry and the Pirates and then about the first two decades of Steve Canyon.