Diazonium salt contains the diazonium group of N triple bonded to another N. This group has a tendency to removed as N2 gas (N2 has triple bonded nitrogens). The electron donating nature of alkyl group supports this removal of N triple bond N as N2. So alkyl diazonium ion is not stable. However, the electron withdrawing nature of benzene prevents diazonium group from being released as N2 gas. So aryl diazonium group is stable.