Because while five of the eight Dipper leadership contenders are wasting their time in a little-watched debate that no one cares about aside from their own supporters, he's out shaking hands and getting votes in the East Island. To point out, so was Paul Dewar, except in St. John's.
Seriously, the only reason Topp and co. wanted Mulcair there was to throw a few barbs his way by attacking the "right-wing government" of Jean Charest (which Mulcair was a minister in). That's about the extent of the interest I think anyone had in the debate. To see that little spark.
These debates are stale, they're full of utter agreement, they're boring and there's too many of them. Lord knows I agree with the concept that the more the voters get to see of your candidate, the better the chance of getting your message across, but this is ridiculous. Who watches these things? I don't anymore.