Why do most biologists consider rna to be life's first coding system?

Because they studied a lot about dna and rna and figured that rna was the first coding system of lofe

Some RNA sequences appear to have changed very little through time. Many biologists consider RNA to have been life's first coding system. ... Evidence indicates that a sequence of chemical events preceded the origin of life on Earth and that life has evolved continuously since that time.

It's either -limiting factors or -scarce resources but i'd go with my first answer since the food and water are abundant they really aren't scarce