Because the Roth IRA is better, especially at higher income levels.
At lower income levels, and I have no idea what the cutoffs are, you can put pretax income in a regular IRA. That means you don't pay income taxes (but you do pay SS and Medicare taxes) on your contribution. But when you take money out of the regular IRA decades later in retirement, you pay income taxes on the withdrawal.
In a Roth it is reversed. You pay income taxes on your full salary, and your Roth IRA contribution does not reduce it. But, when you take the money out it is gloriously tax free.
Obviously a lot of factors go into the calculation, but unless we eliminate the income tax or reduce it to something negligible, the Roth benefit of tax free withdrawals is almost always more valuable, especially with a long time period before retirement, even though it just hurts more when you are young and saving.
But, at higher income, it's not even a trade off, because when you're above the income threshold and use a regular IRA, you have to pay income tax both when you first earn it and again when you withdraw (you get a partial credit for the contributions -- super simplified example: if you contribute $100k over the years and with cap gains, dividends and interest it is worth $300k when you start withdrawing, you pay taxes on 2/3 of every withdrawal, but it's actually really complicated). Since you have no choice but to pay taxes when you contribute you definitely want to convert to Roth and get the tax free withdrawals.
Finally, if you think: hey I'll use regular IRA when really young because I have no cash on hand and I want to reduce my initial tax bite, and I'll switch to Roth a bit later, well, just remember that I said "It only really gets tricky if you first build up a bunch of funds in a traditional IRA..." Because then converting to Roth, even just for your new annual contributions, gets both complicated and expensive.