When estimating the bad debt expense why do you have to create the contra asset account allowance for bad debts why cant you just credit the accounts recivable account?

Since bad debt expense is just an estimate, it is possible that the entire A/R will be collected. Therfore, you still need a record of the gross amount. The expense simply creates and allowance/reserve against it. Also, it gives users of financial statements more information when they can see the gross recievables and how much is not expected to be collected. It can be compared over time, with other companies, etc.