When you're dealing with such small companies, they're usually pretty early in their market and might not have built up enough defensibility to share all of their information publicly. They'd rather limit disclosures to those who have taken some action to express interest.

Public listings increase the number of copycats the end buyer will have to deal with. This isn't much of an issue for a well-established company with significant market share, but it is for a young software company fighting it out with their other (early) competitors.