Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).
Second, zircon has a high trapping temperature of 900°C.
Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.
Uranium easily substitutes for zirconium while lead is strongly excluded.