why the reactivity order of sn1 and sn2 are inverse of each other?

Dear Gopal,

It is both but mainly the stability of the carbocation (I<II<III) for SN1. If the compound is to bulky the nucleophile won  t be able to come in for SN2.

SN1:2-bromo-2-methylpropane (III)

SN2: 1-bromobutane (I), 2-bromobutane (II), bromocyclohexane (II), bromocyclopentane (II), 

The IIairy cation can also give SN1 reactions especially if there is a good leaving group (Br>Cl) or if the carbon atome is hindred (cyclobupentane>cyclohexane)

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