...if you know can you answer, and also hazard a guess as to why, thanks.

MR is the most commen. ~2% of the general population have moderate MR. The prevalence of AS is only 1.3% and thats in people aged between 65-75 (it increases with age, but is much less in younger populations). The reason why, is most likly because the (once) most commen cause of AS ->rheumatic heart disease is hardly ever seen in developed countries(aside from in indiginous populations) anymore.

MR (mitral regurgitation) is much more common than AS (Aortic stenosis). Mitral valve prolapse which can cause MR is a common condition. There are many other causes for MR, while there are much fewer causes for AS.