To answer your question in very simple terms govindarajaa, the reason Krishna is so great and there is much emphasis on his devotional service is because He is the supreme personality of Godhead!
There's no difference between Rama and Krishna.
Yes families tend to have particular deities that they worship due to tradition, namely Shiva, Parvati, Hanuman - but one should consider that without the sanction of Lord Krishna such service to demigods is not possible as stated in text 21, Chapter 7 of Bhagavad-Gita as it is.
Krishna says: "I am in everyone's heart as the Supersoul. As soon as one desires to worship some demigod, I make his faith steady so that he can devote himself to that particular deity"
Krishna also explains why demigod worship isn't the best either...
You should read the Srimad-Bhagavatam for more information or the Bhagavad-Gita. Swami Prabhupada's translation is excellent.
indulekhadasi's answer is great.
