Why they do not send my work to referees while they are unable to conclude that the work provides the sort of firm advance?
They do not send your manuscript to reviewers because they consider that the chance of it being accepted (with revisions) is very small, so that it's not worth reviewers' time.

Their phrasing is just a polite way of saying that the manuscript you have submitted is either clearly out of scope, or clearly not good enough. Maybe you are aiming for a too prestigious journal, or maybe your work is not novel enough. It is also possible that your actual work is very good, but that your description of your work is poor, so that the editors concluded erroneously that it doesn't show much advancement. It is not possible to conclude in general what the true reason is.

If you have no experience in submitting manuscripts to scientific journals, and you have no cooperation with somebody who does, getting a manuscript published is quite difficult. If you don't already, I would suggest you strike up a cooperation with someone who has experience in publishing scientific articles. They could advise where to publish and how to formulate conclusions.

Edit: Also read Benoït Kloeckners excellent remark below: indeed it is your advantage if you know quickly that your paper won't be published, rather than only after a time-consuming peer-review.