It stems from the restriction on IRA. You have to understand some historical context here.
IRAs were created by the Congress in 1974, as part of the ERISA. At that time, most middle- and upper-class employees had pensions. Those who needed the IRA benefit were the poor. In order to limit the benefit to those who needed it (and had no other way of retirement savings), the income limits were put in place. When the Roth option was added in 1998 - there was no overhaul on the IRA system, they just added another option. So they also kept the idea of limiting who can benefit from it, though there's a "backdoor" to work around the Roth IRA income limitations.
Since 1974 pensions became less and less popular, and are now almost extinct. Instead of removing restrictions and raising the limits on IRA, the congress came up with yet another retirement savings vehicle: 401k. So IRA became a weird thing on the sidelines with very low contribution limits and MAGI limits on availability that don't really make any sense in the current economical landscape.