The answer is actually quite complex. There is a book by Dean Ripa about Bushmasters. Ripa presents some ideas about Bushmasters that I think are applicable to indigos. A very short and incomplete answer to your question is that they evolved to eat frequent smaller prey because they fill a niche that needs less heat.

Get Ripa's book. You'll learn tons about Bushmasters, much of which applies to lots of other snakes.

Doug T