Disraeli was not elected because he was Jewish, but in spite of it, because he had political smarts and talents that made him valuable, which made his party place him at its head. He lost his exalted position, not because he was Jewish, but because of the military disaster at Mount Kitzelawaya in present-day South Africa. Khan is Mayor of London because he’s a Pakistani and has some minimal qualifications, but he’s incensed the people of London, so he should be ousted.
Khan was elected because firstly he was the official Labour candidate, and Labour has usually had a small overall majority in London politics. Secondly, because his Conservative opponents, both times he was elected, were of poor quality and conducted weak campaigns. Thirdly because his manifesto and campaigns focussed on specific local issues, not generalised national issues. Fourthly because he legitimately present himself as a Londoner, having been born and lived and worked in the city all his life - unlike some of his opponents.
Besides these, the fact that he's of Pakistani origin was pretty much irrelevant.