If i have a grown dependent living with me I pay 90% of all household expenses why do I not qualify for head of household?

I'm not sure, but I think there's a question that asks if the dependent (not you) paid more than half of their support. Many do not interpret that question correctly and answer it YES. That disqualifies you from claiming HOH.

The basic rules:

You might be able to claim head of household (HOH) filing status if you meet these requirements:

• You’re unmarried or considered unmarried on the last day of 2022.
• You paid more than half the cost of keeping up a home for the year.
• A qualifying person lived with you in the home for more than half the year. Temporary absences, like for school, don’t count. However, if the qualifying person is your dependent parent, the parent doesn’t have to live with you.

To be considered unmarried on the last day of 2022, you must meet these tests:

• You file a separate return.
• You paid more than half the cost of keeping up your home for 2022.
• Your spouse didn’t live in your home during the last six months of 2022. If your spouse is only temporarily absent, your spouse is considered to live in your home.
• Your home was the main home for more than half the year for your:
  • Child
  • Stepchild
  • Foster child

You must be able to claim an exemption for the child. However, you meet this test if you can’t claim the exemption only because the noncustodial parent can claim the child. In other words, you have to be the custodial parent.

It sounds like you should qualify.  Do you claim them as a dependent?

To qualify as a dependent relative:

• They are not claimed as dependent by anyone else.
• They are US citizens.
• They live with you? They must live at your residence all year.
• They make less than $4,400 in 2022? 
• Do you financially support them? You must provide more than half of your relative’s total support each year.

To qualify as head of household, you must:

• Pay for more than half of the household expenses
• Be considered unmarried on the last day of the tax year, and
• You must have a qualifying child or dependent.

This tax filing status includes single parents and divorced or legally separated parents with custody of the child. Further, you can also be an adult who provides support for a parent or other relative under qualifying circumstances.  also see Head of Household 

@rebekahgreer You have not mentioned whether the dependent you are claiming is related to you.   In order to file as Head of Household your qualified dependent must be blood-related.  Is that the case?   If they are not related to you, they cannot be your qualifying dependent for HOH filing status, although they could still meet the criteria to be claimed for the $500 credit for other dependents

Am I Head of Household?

https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household

https://ttlc.intuit.com/questions/2900097-what-is-a-qualifying-person-for-head-of-household

If you qualify as Head of Household, when you enter your marital status (single or married filing separately) into MyInfo, and then enter your qualifying dependent, TurboTax will offer HOH as your filing status.

CREDIT FOR OTHER DEPENDENTS

https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-credit