Ward didn't want to pay the extra sanctioning fees for a belt he was never going to defend. Dawson tried to negotiate it in, but wasn't going to fuss about it because at least this way he kept his belt even though he lost. Had they decided to do so, they could have easily weighed in twice for the same fight like they did for Leonard-Lalonde and put both sets of titles on the line. Just put a shirt on for the second weigh-in, and then you're fighting at LHW. If you go back and look at the news stories from my site back when this was being negotiated, you'll see that all of this came up then, and it was a choice Ward made not to have both titles up for grabs.
What you are asking IS correct, and the fight COULD have been for the 175 titles too, judging by the posts a lot don't understand the rules. As long as a champion and challenger are entering the ring BELOW a weight limits maximum allowed the titles should be on the line. Doesn't matter how far below, as long as they don't go over said weight limit's ceiling. Now as to why they didn't in this fight, I truly don't know unless they must have agreed up front that they were to be left out :conf