Stolen Endymion

Here's the stolen Endymion by George Frederic Watts (c.1872). It's the only one in a private collection, so it must be the one stolen from the Bulmers (next post down). It's already in poor condition with oil paint flaking off. If the thieves treat it roughly, it may become damaged beyond repair. Why was such an important British artwork ever allowed to fall into private hands?