@glemon Some of the impetus was that Chrysler's plant in Linwood, Scotland was seriously struggling, the factory had been built in a logistically impractical location in the first place to satisfy government pressure to spread automotive jobs around to more areas of the UK, and had always been kind of a money loser. They spent a fortune retooling it for the Avenger, expecting that the car would sell well enough on the UK domestic market to justify having two plants build it, but results were disappointing, so shipping some of them to the US was a way to try and keep the Scottish plant viable.
Plus, of course, the Avenger was Chrysler's own product that they fully owned all the way through, R&D, intellectual property, etc, so might as well make use of it if North America wanted a new small car, and selling different models under the Dodge and Plymouth brands helped differentiate them a bit, which had been an increasing problem for some years by the early '70s.