Because they may - and should - want to sell these models simultaneously.if it were to be the same form factor as C100s, why not call it C100 MK III ?
Next comes marketing/corporate synergy stuff. Do they lower C100 MKII to ~ $2,500-$3000 while sticking C200 at $6,000-$7,000? Do they lower C300 MKII to $10,000? Do they come out with C300 MKIII or C300MKII SLT? Do they offer C100 MKII and/or C300 MKII in a package with a bunch of lenses and/or accessories to not completely blow up those sales while there's plenty of inventory to clear?
To give some credit to the executives, most of the time they have Plan A, B and C worked out way before the slip hits the hay.