If I am a Maryland resident of 2019 and 2020 but worked in Colorado and paid all Colorado state taxes, why do I owe Maryland almost 1k this year?

Your state of residence can always tax all of your income, regardless of where you earned it.  Other states can tax you as a non-resident on income you earned by working within their borders.  But in these situations, your home state will credit you for taxes paid to the non-resident state.  You will then owe your home state only the balance due after the credit is applied.

In order for TurboTax to calculate and apply the credit correctly, be sure to complete the non-resident state return first, before you do your home state return.

It depends.

Maryland taxes residents on all income if you meet the filing threshold. (See page 7 in the link below for the threshold amounts.) 

MD Filing instructions with filing threshold amounts

It does not matter where the income was earned.  

Please note, you should be able to claim a credit for any income earned in another state that is also taxed by Maryland.  This ensures you are not taxed on the same income twice.

I recommend you review the following to ensure your returns are calculated correctly.

In the "My Info" section of TurboTax, confirm that you have entered the correct state information: 

1. Click "Edit" to the right of your name 
2. Confirm your input under #2 "Tell us the state(s) you lived in" 
3. Do the same for your spouse if you are married filing jointly. 
4. Go back to the personal information section, and verify the mailing address and "Other state income" section.  Be sure to answer yes to the question asking "Did you make money in any other states?" to report your Colorado income. 

This will then allow you to enter your state returns for the nonresident state as well as the resident state. 

Please note the following as it will affect the calculations: 

Make sure you prepare the Nonresident Colorado state tax returns first.   Be sure to answer the questions correctly to ensure only income sourced to those states specifically are reported.

Once you have entered all of the nonresident state information, then start your Maryland return.  This will ensure you receive any credits for taxes on income which may have been taxed in both states.