Outside of the United States, the accordion is very popular. In the US there is a stigma associated with polkas and other "unhip" genres. Many people also find it difficult to correctly play the instrument. Overall, the sound doesn't have much commercial value or mainstream appeal like the sound of guitars or keyboards.
The accordion also doesn't blend well with other instruments, and it sticks out like a sore thumb. The instrument also has a minimal range. The accordion is never heard in music that is typically considered to be popular.