Ramen is a more complex and heavy noodle soup than pho. Fans prefer ramen because of its complexity and richness. The most important part of ramen is the broth. It's made in an exacting and long process—if the broth is no good then the bowl of ramen is ruined.
The next most important part is the noodles. The noodles must be cooked to just the right point; overcooked and soggy noodles will ruin it. What gives ramen a leg up over pho is the variety and customization available to diners. Ramen has grown far beyond its roots and has evolved as it spread across the globe. You can basically put anything in it.
Pho's secret is its simplicity. A good bowl of pho does not need any additions—the quality of the base ingredients should be more than enough. Pho's fans prefer it over ramen because of its simplicity—the individual components are allowed to shine. The broth should be rich and clear, but never heavy. The noodles should always be fresh and the meat should never take precedence over the other ingredients. It's about balance.
Pho has less variety than ramen, but that's not a bad thing. With pho, less is more.