It seems to just be cost and possibly the difference in cubit feet of storage space? . Cost of the new truck, cost of training and cost of maintenance (Through the life time of the truck). I can see it being cheaper to purchase, equip and train firefighters on 4 rear-mount tower ladders then it is on 4 Rosenbauer T-Rex's for example or 4 Bronto Skylifts.
If we look at what CFD has had to do in recent years with budgets - removing apparatus, deferring Stations, ff's, etc.. It makes sense for them financially to replace their Tower Ladder fleet with apparatus that are a little less expensive but still do an amazing job and will be less cost to maintain and train on.
Don't get me wrong, I LOVE Articulating Aerial Platforms and they seem so valuable on the fire ground but i'm not the purchasing committee
Also, CFD seems to really like Pierce Aerials. And if you notice with the new Smeal Engines in the city - they're 2 1/2 feet shorter then the older 2010 Rosenbauers. So a smaller apparatus foot print is something CFD wants and they might just get them with Pierce -
Pierce - Ascendant® 110' Heavy-Duty Aerial Platform | Pierce Mfg