What did I do wrong?

You did nothing wrong, but may not have finished the process. To use the STEEM which you purchased effectively you will need to power up that steem. That can be done in your Wallet by using the drop down menu to the right of your STEEM amount.

By powering up your STEEM will give you a greater stake. This is what can make one person's upvote more valuable than another person's due to them having more stake.

It does take a number of weeks to power down your SPs (Steem Power) to be able to use it again as currency.

STEEM On!!!