Why? They were synonymous for five decades!
Synonymous because conservatives were more hawkish. If your big thing is getting involved overseas, you'll associate with whatever party supports that.
I look at it similar to abortion. If you are militantly pro or anti, it becomes very easy to associate you with a particular party. But if either party softens their view, a bunch of people would flip parties because their non-abortion political views align better with the other side.
For Tom Nichols, foreign policy stuff is his one true love. Democrats are similarly hawkish (maybe more?) to the GOP. So there is no need for him to be in the GOP. He can have his cake and eat it too by supporting democrats or even just opposing the GOP. He gets similar foreign policy with his preferred domestic policies.
If Dems were to become doves again, and President Tom Cotton was leading a warhawk GOP in a standoff with Russia or China, Tom would be cheerleading and writing articles about how the GOP is back in his good graces and downplaying his domestic disagreements as minor issues.