Things are just different. You can't match things perfectly. When you bring them in, the system is probably a little bit faulty in itself. Both players came in and got hurt early. It was just one of those things that didn't work.
Wide receiver is one of the trickier offensive positions when it comes to switching systems.
If a team brings in a free-agent quarterback, they'll likely change or at least tweak their offense to better suit his needs. If a team brings in a top free-agent running back to handle most of the rushing duties, it won't affect a team's system too much, unless they decide to put a greater emphasis on the running vs. the passing game. The quarterback is still going to hand the ball off, the offensive line is still going to be pushing against the defensive line to create holes. And he has to adjust to his fellow wideouts, up to two or three of whom will be lining up with him on every play. Staying with the team you've been with then, can have great benefits.