If you've self-published then you've already tested your book, as a product, on the marketplace. If it failed to sell in any great numbers, then in the eyes of a lot of publishers, you’ve proved that it doesn't have the potential to sell in sufficient quantities for them to take it on. This might well be because you, as a publisher, don't have access to the same sales and marketing clout that the bigger publishers employ, but many of those big publishers aren’t interested in that: all they see is the numbers they find on Nielsen’s sales reports. As far as they’re concerned, your book hasn't sold well and so you've shown that it's an uncommercial product.
This probably means that those publishers miss out on a few books with real potential: but as publishers have so many titles to pick and choose from, it's no wonder they tend to dismiss books so easily.