Why In a parallel RLC tank the resistance appears across the circuit, so a larger shunt resistance means less loss and a higher Q: Q = R / X, where X is the reactance of either the inductor or capacitor (they are equal at resonance). This is the reciprocal of the series case, where the resistance is in line with the reactance and Q = X / R. At resonance you can use either XL or XC since XL = XC.
Watch out The choice dividing reactance by resistance is the formula for a series resonant circuit; mixing the two up is the classic trap in this group.
Parallel: R is on top (R/X). Series: R is on the bottom (X/R). The R follows where it sits in the circuit.