I thought facades were usually retained (and built behind) when the facade (but not interior) was listed, or in a conservation area where the facade is considered to be a material asset to the character of the conservation area. The Pool isn't considered worthy of listing (by English Heritage) and isn't in a conservation area.
Keeping only the facade would impose considerable practical and financial restrictions on the future development of the pools, as well as producing a final building that was neither a great Victorian building, nor a fine piece of contemporary architecture.