Oh, I don't know, it could be the fact that a humble ploughman from an impoverished background, in a pitifully short life (died at 37) wrote a huge amount of incredible poetry and gathered, rearranged and rewrote lyrics for many traditional Scottish songs, including possibly the most famous song and most sugn song in the world - Auld Lang Syne.
Although his personal life would raise eyebrows even now (it certainly did in the 1780s and 90s) he wrote about humanitarian ideas that are relevant today and were startling at the time he wrote them.
No other poet is celebrated the world over in so many languages and by so many diverse communities. There is, after all, no Shakespeare Day or Wordsworth Day.
If you don't "get" Burns, why not try reading some of it (I assume you haven't). Most books have a glossary of the more obscure terms to assist the reader (heaven knows, I need it and I live in Ayrshire).
Try this site for an English translation of another of his most famous works along with an explanation of what it's about.
(With thanks to my dh, the Burns aficionado/fanatic)