I have not ignored your comment on Anchors Away but have not yet heard from the local historian to whom I addressed your query to. My initial feeling was that the reason for Rees using horses rather than traction engines to move large castings was because that is what they had. However I have enquired of an eminent Black Country historian whether he is aware of any underlying reason for this situation.
|Clydesdale horse at Kinver Fair|
On water, a horse can pull perhaps 50 times the road weight.
|Towpath tunnel by Wolverley Lock|
Although Clydesdales are generally smaller than Shire horses I think they are also too large for many of the Midlands canals - particularly the Staffs and Worcs where towpath tunnels can be very low.
This picture from the HorseBoating Society shows the likely size of a canal towing horse.
(Often pairs of donkeys were used in the 18th century.)