Why a rotating lift-dock rather than a conventional long series of horizontal docks?
Land use, land price. Excavation fees and "where do I dump the excavated dirt?" problem. (No Gatun Dam handy, unlike Panama.) Going up that far takes a considerable horizontal distance AND greater regional impact going down the slope for the two sets of locks. Also, more water goes through each lock for each ship transit - since each conventional lock requires twice the depth that a single level basin requires. Worse, a conventional lock requires the "upper" water to be lost for each transit. With this style, the water is transferred back "uphill" with almost no loss in the upper reaches of the canal.
And, never ignore the "It looks pretty" factor. If the item can work as a different design, is economical (can get funded!) and "Can get sold" as a feature or an attraction for people to enjoy - rather than get opposed by endless enviro lawsuits and be an "ugly concrete ditch" going through the neighborhood - .....
Why not build it that way?
Enjoy two (or three, or four!) working solutions rather than a 1970'd ugly piece of government-paid square concrete box