It's the plurality national language (there's no single language with a greater adoption in Ethiopia) but it's not the majority language -- fewer than 50% of Ethiopians are able to use Amharic. So, "most people" in Ethiopia use a local language instead of Amharic.
I am actually from Ethiopia, and from what my parents who were raised there tell me, the education was in Amharic so they learned to read/speak it early on, alongside their morhertongue Somali. Of course ethnic groups tend to use their language first and Amharic for more official matters. So I don't really see what the problem with using Amharic and English is. You're reaching more kids and like another poster said, there's more resources and apps available in Amharic than Afaan Oromo.