Democrats are hardly predictable in who they will nominate for President. Democrats will play the hot hand so we end up with candidates like Carter, Dukakis, Clinton and Obama
Republicans never think outside the box so we tend to get the next in line guy
Ummm...both parties work the same way, the party elites pick whomever they think is electable and can be locked in to whatever policies the party elites want them to support once they figure that out that's who gets nominated. All those Democratic nominees you cited fit the pattern, same goes for the Republicans. The only difference is that on occasion the Democratic Party elites have taken longer to figure out who fit the bill, they still come off the short list though.
Like I said look for the biggest douche bag (i.e. the one that will sell out completely to the party elites) and that's who'll get the nod in either party.
This election cycle *might* be a bit more interesting since the Democratic Party elites picked Hillary early on and that's turning into a boondoogle for 'em and the Republicans have a big shake up from outsiders that they can't control.
Hopefully this will time out will prove so frustrating to the elites in both parties that they'll do the world a favor and start jumping off tall buildings.... but I wouldn't count on it.