There is a general criticism on Srila Prabhupada that he only talks about Krishna and hence sectarian. This criticism is very common among mainstream "hindus" and Indians. This criticism also reveals the lack of proper philosophical understanding of the concept of Godhead and His concomitant material creation.
There are two ways to arrange things. One way is horizontal and the other vertical. When we arrange things linearly in horizontal fashion, then it seems all of things arranged have equal weights thus demanding equal attention from the viewer. If however, we adjust the heights in the vertical direction, then the top aligned items are given more importance. The same can also be applied with size. Two identical objects can be sorted if one is bigger in size than the other.
The misunderstanding about Krishna is that the general "hindu" population assumes Krishna to be on equal footing with other Gods like the horizontal arrangement of things as mentioned above. But actually Krishna should be seen vertical in that He is on the top and the others below Him. If we see Krishna with this arrangement, then by simply worshipping Krishna, all other subservient entities are included. One need not worship other celestial entities separately.
So when Srila Prabhupada focuses purely on Krishna as the object of worship, he does so because all other 330 million gods including Lord Siva and Lord Brahma are included in Krishna worship. One need not separately go to a Siva temple and worship Him. Of course to understand the super-inclusive nature of Krishna requires purification from mundane desires and certain amount of faith.
Srila Prabhupada therefore is not preaching sectarian dharma but a super-dharma which includes all other subservient forms of dharma.