It's not quite that simple, it really depends on what you suspect your tax rates are going to be. Roth contributions are essentially taxed at your marginal rate right now, but traditional 401k withdrawals are taxed like income when you make the withdrawal.
There are lots of situations where you would want to lower your tax burden now instead of go for the Roth.
Note that you can also have a Roth 401k or a traditional IRA.