publishing excerpts

I just received this rejection from an anthology editor for the first chapter of a WIP I had submitted:

I’ve gone back and forth on your submission but have finally decided not to include it. This is not a reflection on the quality of your submission. This is part of a novel, and I want you to complete your novel and shop it without worry that an agent will reject it because it has been published elsewhere. Best of luck with that process.

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Would an agent reject a novel simply because an excerpt had been previously published? (This might explain my lack of success with my other two novels.) I've been under the impression that published excerpts were a good thing, that it showed that others were interested in it, that it was a kind of pre-publicity. Wrong?

folkchick said:

Perhaps their guidelines are such that they own copyrights for the life of the work, thus leaving you unable to include the piece in a completed novel. It sounds like they are thinking ahead for you.

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I should've mentioned that their guidelines state:

All rights revert to the author after publication.

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I always look out for which rights they're after. Nicest rejection ever? I don't know, I guess I was too confused to notice it that way. Thank you both for your help!