Hare Krishna ! Please accept my humble obeisances ! All glories to Shrila Prabhupada !
Many people have raised a question that why does our Guru Parampara tell everyone that Krishna is greater than Lord Shiva ? The answer is that Padma Purana warns the devotees of Lord Krishna to be careful against the ten offenses to the Holy Name otherwise they will not be able to get the fruit of their chanting i.e love of God. One of the offenses is to consider Lord Shiva equal or greater than Lord Krishna. Therefore our guru paramapara has been educating the sincere devotees that Lord Shiva is subordinate to Lord Krishna to save them from the offense.
Following is from Jaiva Dharma-
Vijaya-kumara: O master, what result is obtained by namaparadha (offenses to the holy name)?
Babaji: Even if they were multiplied millions of times, the five sins would not equal an offense to the holy name. You can very easily see the result reaped by offending the holy name.
Vijaya-kumara: O master, a person who offends the holy name certainly reaps an inauspicious result. Still, does he not also obtain a good result because he has spoken the syllables of the holy name?
Babaji: The holy name fulfills the chanter's desires, whatever they are. However, to the offensive chanter the holy name will not give pure love for Lord Krishna (krishna-prema). Eventually the offensive chanter must taste the result of his offenses. However, a person who, although he commits offenses to the holy name, regularly chants the holy name will attain the result of his chanting. After some time his chanting will become a great transcendental pious deed. As that transcendental pious deed grows greater and greater, he will attain the association of saintly devotees who devotedly chant the holy name in a pure way. Thus by always chanting the holy name, the offender eventually becomes free of his offenses to the holy name. By following this path even impersonalists gradually become sincere devotees of Lord Hari.