When SNPs occur in coding regions, the majority are deleterious/pathogenic/harmful and either not compatible with life or sadly, affect survival to reproductive age. Therefore such SNPs are not propagated to offspring/future generations.
SNPs in noncoding regions may have no effect or only mild effect and therefore the individual survives beyond reproductive age, passing on the SNP in the noncoding region to offspring. In this way, SNPs in non coding regions can accumulate over generations.