There is much talk of late about the need to downvote rather than close or delete certain content. I see these come through the low quality review queue frequently. To give them the appropriate treatment, I have to make several extra mouse clicks to open up. It would seems that you would want to optimize and encourage the correct behavior of downvoting by allowing downvoting right there in the LQ review framework.
I don't think this should be a top priority since many posts that end up in that queue are not an answer posts that are going to end up being deleted anyways. Probably like 40%? Just a guess. Depends on the site.
If a question makes it to the LQRQ chances are it already has -1 score or worse, and a close vote from you is top priority. If you want to help out as much as you can, then do the extra clicks and downvote it too.
The only reason I can think of why they don't have it included already, is that they don't want people to just downvote and pass the review successfully, and then have to click back to go and close vote it too. So some technical thing.