Because you might not enter it in the right place. Here are the steps:
In TurboTax online,
Sign in to your account, select Pick up where you left off
At the right upper corner, in the search box, type in "1098t" and Enter
Select Jump to 1098t
Follow prompts
If you have paid tuition expenses to your school, your school will issue you a Form 1098T. Report the form as it is. If you have received any scholarship, and if it is used to pay for all of your tuition expenses including books and supplies, you will not need to pay tax on it. However, if you received the scholarship which exceeds your qualified education expenses, you will report part of your scholarship as a taxable income. To see what qualified education expenses, click here:
Assuming you're asking about a 1098-T (tuition) and not a 1098 (mortgage interest);
Go through the entire education interview until you reach a screen titled "Your Education Expenses Summary". Click edit next to the student's name. That should take you to a screen “Here’s your Education Summary”. Click edit next to the section you want to change; tuition, in your case.