Actually, no, they really didn't. As much as I would like to pretend that this move wasn't financially prudent, it really was.
First off, it is well known that core book sales are the brass ring of the D&D market. Supplements, adventures, etc., do not compare.
WotC already sold near as many copies of the core as they were going to sell, barring the occasional replacement or new entry. So, sales of core books probably had slowed to a trickle. To rejuvinate these sales, they had to produce new books that had markedly different material, or people wouldn't buy them.
Further, owing to the structure of the d20 license, d20 product sales help drive core book sales. So, by making the core book more different than was expedient for the needs of the adience, they also create the situation where the new d20 books based on the SRD are different enough that the existing books are not sufficient to use the existing supplements. So by making the books more different, they help drive the sales of the new books indirectly.
This is good for WotC and good for d20 publishers. For the audience, on the other hand, the situation is less than optimal. But that's the nature of the hobby. By their very nature, RPGs are low maintenance, so you have to expect that the businesses that stand to benefit from it are going to try to change that situation to drive more sales.