Do you really?
I think it is currently the second best selling debut album of all time and every track on it still gets lots of airplay.
I imagine all the Elvis Costello fans among the rock journo fraternity don't rate it but it's highly rated by those that like AOR/Hard Rock.
Obviously it's more highly rated in the States than here but I think that making a case for an album that has gone platinum seventeen times being underrated is quite the stretch.
Mind you, it depends on how you define underrated: compared to my rating of the album, most people could be said to underrate it. I think it is an absolutely superb album; perhaps the most perfect example of it's (sub-)genre.
I think the follow-up "Don't Look Back" was underrated because of the shadow cast by its predecessor.