First of all, I assume you that you know about a bit about ten incarnations of Lord Vishnu. So you obviously know that both Ram and Krishna are the incarnations of Lord Vishnu. The only difference is that Ram came first. After departure of Ram, Krishna came to this world. You don't wonder the story of Krishna is more popular than Ram. Yes, Ram and Ramayana also is not less popular. But the story of Krishna is so interesting and inspiring that people never feel tired of listening the pranks of Lord Krishna.
Lord Krishna is popular specially with the pranks he has performed in his childhood. Assassination of Putana, War with Kaliya Naag, event of lifting Gobardhan Parvat to save people, naughty behaviors with Gopinis, love with Goddess Radha, Assassination of Kansa, etc happened while he was a child. Except Mahabharata War and Preaching Geeta, he completed his almost all major missions in his childhood. This is the exact reason, the naughty child Krishna is depicted in idols to worship than the grown up Krishna.
Lord Ram also did so many pranks and duties in his childhood. But his actual mission was fulfilled when he grew up. Assassination of Ravan is one example. So the Big Ram is famous among us than the Little Ram. Now I hope you understand why naughty, childish Krishna is popular and the Big Ram is great.
Discovered by +Don Prince