>Is there any way to find out the reasons for this or maybe get some assistance for submission?
Very likely there is, but the editors don't offer it.
Since the guidelines are public, anyone can read them and see what kind of sites the ODP lists. (Keyword: "unique content".) You can ask any surfer of your acquaintance if they can find unique information on the site. If they have trouble doing that, then you know where to start.
You can also use Google to see how many sites there are similar to yours. If there are more than a few hundred thousand, then it's very possible that editors have gotten around to looking at it: there is a large probability that MANY sites will need to be looked at first, and a vanishing probability that there will be anything new found in that search -- so editors may have been working in other, less over=served areas. This isn't exactly rejection but (of course) has the same practical effects.
As for submittal assistance, the editors have submitted millions of sites without being asked. We don't offer help with submittals: we ACCEPT help via submittals!
That's right, you can help us find sites. Without your help, we'll do just fine -- there are plenty of sites we already know about, or know how to find, and we'd spend our time happily finding, reviewing, and listing -- whether YOU help or not. Everything we do is geared toward helping surfers find sites.
But there is no way you can "get a site listed." That part of the work is reserved for editors; and it is just flat not a service offered to webmasters.