As we explained on the Devils board, it's a combination of a couple things. Anaheim not necessarily wanting to give out a long term deal when they have a bunch of young D who'll need new contracts in a couple seasons. Combine that with Anaheim not exactly being a hot free agent destination. You keep making it seem like they should have been a contender for Brett Pesce if they had offered one more dollar than the Devils. Who else was on the market? Montour? I don't think that's somebody the Ducks wanted a 7+ year reunion.
You even went as far as saying they should have signed T.J. Brodie and Alec Martinez which doesn't seem like a particularly great solution especially knowing that Martinez has missed more than half the season because of injuries. While both have experience playing the right side, Anaheim was looking for a RHD. Trouba fills a spot for the next season+ and Anaheim can likely get much more than the 4th if/when they trade him at next year's deadline.