Why might noncoding regions of dna outside of genes be more variable than coding regions of dna?

In general, coding regions are more likely to have the same sequence. It is more likely to produce a non-functional gene product (protein).

Non-coding mutations are free to mutate because it has no protein product, and are less likely to affect phenotype. They are neutral mutations. Different mutations can be differentiated even when different groups have the same coding sequence.