Craig MacTavish was asked about Peckham in his press conference this morning, and the exchange went like this:
Reporter: Will Peckham take all of Smid’s minutes, or is it going to increase the workload for the rest?
MacTavish: No, it’ll increase the workload for the rest. With Smid and Strudwick there’s a real comfort level defensively with those two. You don’t want to put a young defenseman in a position where he has to go against the top line or even the second line. (italics mine)
That's why the player behind Peckham is Ben Guite, who has four goals on the season and posted a career-high 22 points last season. Because matchups matter, and for a young defenseman the best place to start an NHL career is on the bottom rung - playing weak opposition.
It's also a big part of the reason why teams breaking in a bunch of young players go nowhere fast - there simply aren't enough 3rd and 4th-line forwards to match up against, so somebody ends up getting burned.