The reason ratings are down is because there is more access to more viewing options than ever before in most homes. With more available options, you should expect a decline in viewership across the board.
Seven years ago, the average household had access to about 130 channels. Two years ago, that number was nearly 190 channels. It's only going to continue to go up in the future.
I couldn't find this data, but I would bet that if you were to go back 15 to 20 years ago, the number would probably be closer to 40 to 50 available channels in the average household.
However, in NASCAR's primary demographic market (the more rural southeast region), there is probably a higher prevalence of homes without full-tier cable/satellite service compared to other areas of the country. Ten years ago, when every single race was available on Fox or NBC, anyone could watch every single race without much difficulty, even on an antenna that only received the big four local broadcast channels (Fox/NBC/CBS/ABC). Now that more of the races are on cable (FS1 and NBCSN) than not, there are probably lots of average NASCAR viewers that are unable to tune in that would like to do so.
NASCAR has lost some of their primary demographic to cable channels they don't have, while they are losing the more casual viewer to other channels they now have access to.
I also think that you get a decline in NASCAR viewership because the presentation of the product is not satisfactory (there are way too many commercials).