I believe you almost answer your own question. After watching the lecture by professor McClanahan, the Northern States attacked the institution of slavery because it did give Sounthern States some more power and representation in the general government which, in turn, helped Southern States block many attempts by Northerners to pass more mercantilist economic policies. Anti-internal improvment/national bank/protectionist (tariffs) ideals wasn’t what gave Southerners more representation power it was slavery; so why attack the former three when it was the latter?
I hope that helps and makes sense.
Side note: Why the Southerners didn’t simply free the slaves and help them get on their freedom feet (yes, I just made that up and yes, it is very cheesy) so their representation would be legitimate and possibly on their side politically I’ll never know. But, then again, I’ll never understand the slavery mindset anyway.