Most publishing houses specialise. There are very few that publish poetry, and not all publish fiction.
Being specialist allows them to work on the kind of books they like (it’s their money they are spending, and however much you might wish they would publish in your area of writing, it is up to them).
It also enables them to penetrate the market more successfully – they will have built up good links with reviewers and the book trade in their area of specialisation, and the latter will consequently take seriously any titles under their imprint.
So offering them a title when they publish no others in that area would be a hard act to pull off, because they have no in-house expertise in that field. (Don’t assume that you are helpfully extending their range for them; they may not want it extended.)
The Writers' & Artists' Yearbook lists their specialisations, and how they like material to be submitted. Many will not accept unsolicited manuscripts, while some insist on you sending return postage. Remember that the manuscript is sent at your own risk – never send your only copy.
If you found this article useful, you might want to take a look at: