The thing about Eifert is that he's really had just the one very good season - 2015, when he played in 13 of 16 games, caught 52/74 (70%) for 615 yards (11.8 YPC) and 13 TDs. Dwayne Allen had three seasons with the Colts that were more productive than Eifert's next best two. And there's no telling if Eifert can be anywhere near as good a blocker as Allen became this season for the Pats.
And Eifert's 2017 contract was for about the same money as the Pats will be paying Allen in 2018. Allen turns 28 at the end of February. Eifert turns 28 the first week in September.
So, basically, Eifert is taller, but has a bad back. And he'd be starting from scratch learning both the Pats blocking schemes as well as the passing offense as a receiver. And likely for at least a bit more than what the Pat's are currently paying Allen, who already knows at least half of that.