I think the reason of this is because in the Tantras and different Puranas glorifying Devi, the relation between Devi and Vishnu/Krishna is very close.
In Devi-Mahatmya, Devi says that She will be born as sister of Krishna.
In the same book, Devi has been referred to as Vaishnavi Shakti.
According to Tantras, Maa Kali took birth as Sri Krishna.
So its basically establishing equality of these two Principal forms.
The beeja-mantra of Kali is KREEM and of Krishna is KLEEM. As per Dr.Mahanamvrata Brahmachari, both mean the same God by the grammatical rule: ralayor abedah meaning R and L are identical.
But in the Vaishnava Culture, such views are not expressed explicitly.
So in Devi temples we find images of Vishnu but the reverese is very very rarely seen.