It really just boils down to a combination of factors, but the predominant ones are price, specs, and ports against personal expectations. If it was just one of the 3, that would be one thing, but it's the stacking of these factors.
I think a fair number of people expected USB-C/TB3, but only to replace the TB2 ports we had -- so keep USB-A, HDMI, SD Card, etc. Yeah, we had the leaked chassis, but that could have been prior to milling the other ports out, so there was still some amount of hope.
While USB-C/TB3 are definitely the future, it's still early, and some of these consumers/prosumers/professionals that traditionally buy MBPs probably have a fair number of production external devices that demand older interfaces. I'm not talking trivial things like a mouse, more like accessing their 6 month old $2000 disk array, or using their SD card from their dSLR out in the field. Heck, to even connect their brand-new iPhone to their new Mac, they'll have to use an adapter (although I expect future iPhone cables to be USB-C to Lightning, we're not there yet).
Specs, short and sweet, RAM should be 16GB base with 32GB and 64GB options, base SSD should probably be 512GB for the price segments. Now we hear about the lower-performing right-side ports on the 13" model, DDR3 vs DDR4, and the lack of Iris Pro iGPU graphics on the 15" (for the situations where you don't want/need to engage the Radeon card). Compromises along the way on machines that were rarely compromises in the past, spec-wise.
Pricing is easy, really -- prices are a few hundred dollars higher than the models they're replacing. The base 15" used to be $1999, and now it's $2399. The 13" used to be $1499, and now (for all intents and purposes, CPU-wise) it's $1799. They're attempting to give more "value" to command those higher prices, but if you didn't need a dGPU on the 15" for example, you're paying for something you didn't necessarily need/want.
And for clarity, I did pre-order a 15" with the Radeon Pro 460, but I also understood (and accepted) what I was buying as well.