Because that's what a Roth conversion is all about.
With a traditional IRA, your contributions are deducted before tax (you get a deduction). That means you pay tax on the contributions and the gains when you withdraw them. With a Roth IRA, you pay taxes on the contributions, and you don't pay tax when you withdraw the money when you retire. In other words, you have to pay tax on the money going in, or coming out. When you convert a pretax traditional IRA to a Roth IRA, you pay tax on that money going in to the Roth. You now will not pay any tax when you withdraw (as long as you are over age 59-1/2 or wait 5 years.)
