Answer by Niraj Bidawatka
Because you are in ignorance when you say that Lord Shiva is Supreme. Therefore ISKCON has to give authentic and factual information to you. The Bhagavad Gita is the most authoritative scripture in the entire world. It is considered the essence of all the Upanishads.
You will not find a single verse in the Bhagavad Gita, which states that Lord Shiva is Supreme. The entire Bhagavad Gita states that Lord Krishna is Supreme. Therefore Srila Vyasadeva, the author of the Bhagavad Gita addresses Lord Krishna as “Bhagavan”.
Bhagavad Gita means “the song of God”. Everyone knows that Bhagavad Gita was spoken by Lord Krishna and not by Lord Shiva.
Since ISKCON strictly follows Bhagavad Gita, it states that Lord Krishna is Supreme.
Even Shankaracharya, the incarnation of Lord Shiva, has accepted Bhagavad Gita as the highest authoritative scripture and has directed all his followers to worship Lord Krishna. His famous directions are as follows:
bhaja govindam bhaja govindam
govindam bhaja mudha mate
Sripada Shankaracharya addressed his disciples as fools (mudha mate), because they did not accept Lord Krishna as Supreme. Therefore he directs his foolish disciples to worship Krishna (bhaja govindam) because only Krishna can save them at the time of death.