The Android market fragmentation is the well known problem. However, people blame smartphone manufacturers for this situation. It would seem that mobile producers do not want to spend money for maintaining old devices and they prefer to sell new smartphones with the latest Android version. Unfortunately, sometimes the fault lies with a completely different side.
You have probably started to think about delays caused by mobile carriers. This is also very often situation, when newer Android has been already released by the manufacturer, but it is available only for phones bought in stores without any modification from the carrier. However, sometimes Android update can not be prepared because of lack of support from chipset manufacturer.
Qualcomm will not prepare Android Nougat graphics drivers for Snapdragon 800 and 801 chipsets.
This information comes from LlabTooFeR, who is ROM developer. I bet that you have started to think what chipset model you have. You can check this using for an example the AnTuTu benchmark from Google Play store. These unlucky Qualcomm Snapdragon chipset are used, among others, in HTC One (M8), Nexus 5, Samsung Galaxy S5, Samsung Galaxy Note 3, LG G3 and Sony Xperia X3 smartphones. Of course, this is not the full list, but only the most popular mobiles with Snapdragon 800 or 801 chipset.