Where do you get your stats? I read that 40% of all Bernie Sanders primary voters voted for Trump in West Virginia, with similar proportions in most red states.
ABC "Likely Voters" is a sample and there were many samples conducted during that time.
IIRC, Clinton supporters in 2008 formed an organization called PUMA, but it disbanded within hours after John McCain announced his running mate.
Especially how popular Obama was at the time, and how McCain's VP mate energized Clinton supporters to vote AGAINST that GOP ticket, I cannot imagine a nationwide 15% block of Hillary supporters going GOP in 2008.