It's quite redundant IMO to have 2 phablet. Just release the Note 5 with edges, simple and done. Put all your marketing muscle behind one flagship phablet. With that said, man the edge blows away the Note 4. The Note 5 better look incredible or I may consider ditching my stylus, Bleh I'm bummed about being forced to make that decision though.
2 main reasons
1 - Because you can't have a note 5 with rounded / curved edges without causing an issue with the stylus and Wacom digitizer.
Writing on a flat surface is one thing, but writing on a curved one is another.
2 - You can't have a Note device without the stylus or it ceases to be a Note, it's just another large phablet. The Note after all is named after its pen, and it's software is tailored for pen.
Samsung treated the Note 4 edge differently. It's edge was treated as an independent screen reducing the amount of area one physically writes on compared to the normal Note 4. If you took this approach with the new edge devices which is seen as 1 screen rather than two and reduced physical area for writing then all software will have to be re-adapted to make sure app input areas only fall within the writing / pen designated area. It's just not a feasible plan especially for third party apps.
Hence current solution - 2 phablets - 2 SKU's solves the issue
1. Is a Note 5 with large flat screen & stylus and software designed with pen input
2. Is a S6 Edge + with a large curved screen but without stylus with just touch input in mind.