"Why aren't several seasoned and formidable former senior Cabinet Ministers or Premiers contending?"  Your question answers itself.  Senior Harper cabinet ministers like Kenney and MacKay took a look at the federal landscape and decided it was futile.   As for premiers, most of them understand that premiers never do well at the federal level, because they're too associated with pushing the interests of a particular region.  The last premier to become a party leader was Bob Stanfield, 40 years ago.  I don't believe any former premier has run for any federal party's leadership since then, except for Bob Rae, who was arguably a special case because he had switched parties and had spent a fairly long period out of politics.