Why are Mendelian mutations considered rare?
I will focus more on the question of rarity, which the comments have not addressed. Recall that natural selection is operating on the human population. Because of this, bad mutations of large effect (disease mutations such as CFTR in your figure) are very rare, because selection is removing them at some rate from the population (people with cystic fibrosis are unfortunately very sick and are not as fertile as healthy people), whereas mutations with very little effect are not subject to much selection. Paper one and paper two are reviews describing the population genetics in more detail. 

As suggested by commenters, Mendelian mutations are so named because they are single loci of large effect controlling traits (approximately). Common variants, by this logic, must have very little phenotypic effect, because they are not purged by natural selection (assuming that they are deleterious, or disease-causing). 

Rule of thumb: 'Mendelian' alleles can easily be used to predict phenotypes (like disease) and are acted upon by natural selection. 'Common' alleles are individually not informative about phenotype and are thus mostly ignored by natural selection. Not exactly true, but close.