Because people like the assurance of a check, or a receipt, or a payment confirmation.
How do you initiate a wire transfer? Seems like you would need to know a lot about the recipient -- their bank and account number, for starters -- to be successful. Sure you could do it on a mobile device but seems a lot more time consuming than writing a check. Privacy concern as well.
What happens if the transfer fails or doesn't go through, for whatever reason? If you hand the recipient a check, they have physical proof that you intended to pay them, and they know the payer's bank and account number. What recourse, proof, or evidence does the recipient have for a failed wire transfer?
How do B2B transactions work? Is it all wire transfer or do they pay each other with checks? In the USA, a huge percentage of consumer payments have migrated from checks to debit/credit cards (with near-instant confirmation), but companies still use tons of checks via standard postal mail all the time.