Two reasons: 1) Vaporware; 2) Unforeseen delays.
To elaborate, we do not want to promote items that don't exist. I realize no one is asking that we market or promote them, but it could be construed as that, and we want to avoid this at all cost. We will however announce items that we intend to release in stages once a significant portion is complete (e.g. Syntax, Revised EBC, etc.).
There are inevitably delays in projects, especially larger ones. For example, we actually did something like this a couple years ago and announced LSJ as 'coming soon…' on our New Releases page. At that time we were operating based on information we received, and had every intention of it 'coming soon.' However, that didn't happen and we had to go a totally different route with that resource, which meant we didn't release it until almost 2 years later!
So, my advice is just to ask, either by email or here on the forum, especially if it is about a more popular Biblical studies resource that seems natural for Accordance and is available elsewhere in electronic format. Chances are, we're already working on getting it.
I hope this helps to explain things a bit.