Then why did mainline buy regionals?

You might be right, There is no economic incentive whatsoever but I think it helped finance the NWA pilots pension fund:

Contribution of Pinnacle Stock
Northwest reported that the Omnibus Agreement was executed between
Pinnacle Airlines Corporation (Pinnacle), Northwest Airlines, Inc.
(Northwest), Northwest Airlines Corporation (NWA Corp.) and Aon
Fiduciary Counselors, Inc (Fiduciary Counselors). Pursuant to the terms
of the Omnibus Agreement, Northwest contributed Pinnacle Stock to the
Contract Plan. The Omnibus Agreement provided for two contributions to
be made to the Contract Plan on January 15, 2003. An ``Initial
Contribution'' was made in the amount of $41,149,911. The Initial
Contribution was comprised of 1,819,833 shares valued at $22.61 per
share.\3\ The amount of the Initial Contribution is equal to the amount
that was required to meet the quarterly funding requirements under
ERISA section 302 and Code section 412(l) for the Contract Plan due on
January 15, 2003. The Omnibus Agreement also provided for an
``Additional Initial Contribution'' to the Contract Plan in the amount
of $2,671,983 (118,167 shares valued at $22.61 per share).
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\3\ Northwest represents that the amount of shares necessary to
satisfy the required contribution was based upon a final valuation
of Pinnacle by Fiduciary Counselors, relying on a valuation report
prepared by Eclat Consulting. Northwest notes that, while Fiduciary
Counselors received and reviewed valuation information provided by
Morgan Stanley & Co. Inc. (Morgan Stanley), Fiduciary Counselors
retained Eclat to provide valuation services.