From a technical point of view —expanding @Chris_R’s explanation—, when you depend on other hardware manufacturers (OEMs) —Qualcomm for the SoC, others for cameras, etc.—, all of them with proprietary drivers, and Google for the device certification to use their mobile services…:
There’s more detailed and technical information on this #epic topic Android 6.0 Marshmallow / 7 Nougat for FP2? and this Official Android 7.1 <-> LineageOS 14.1
Furthermore, you need to care about the upgrade process for your users, which have personal and critical data in their phones. The upgrade path is infinitely easier from 6 to 7 than from any other random version jump.