Cause it's a lie.
This does not mean that Jews and Christians living in Muslim-ruled lands were killed or imprisoned. They regularly received dhimmi status, which afforded them temporary protection from some abuses, but no permanent status.
Dhimmis in Muslim countries over the centuries typically had to pay discriminatory taxes and acknowledge publicly their status as second-class citizens. They were on the hook for additional sums and had to supply forced labor on demand. They were ineligible for any public office and without right even to testify in court.
Dhimmis were not allowed to possess weapons, marry Muslim women, meet with others on the streets, or ride horses or camels (the two "noble animals"). Dhimmis had to wear special clothes, walk with eyes lowered and accept being pushed aside by Muslims. Dhimmis had to have low doors on their houses, with no lights on the doors.
Some particular aspects varied from age to age and region to region. In the 9th century, Jews in some Muslim areas had to wear on their shoulders a patch of white cloth that bore the image of an ape; Christians, since they ate pork, wore a pig image. In 11th century Seville, Jews were not to be met with the greeting, "Peace be unto you," because they were not supposed to have any peace.
Other edicts affected not just finances but self-respect. A Cairo rule in 1761 was that "no Jew or Christian may appear on horseback. They ride only asses ... " In Persia in 1890, Jewish women had to "expose their faces in public (like prostitutes). ... Every Jew is obliged to wear a piece of red cloth on his chest. A Jew must never overtake a Muslim on a public street. ... If a Muslim insults a Jew, the latter must drop his head and remain silent. ... The Jew cannot put on his coat; he must be satisfied to carry it rolled under his arm. ... It is forbidden for Jews to leave the town or enjoy the fresh air of the countryside. ... Jews must not consume good fruit."