Because it's an important distinction. OP is using "voter fraud" to refer to voters acting fraudulently, and "election fraud" to refer to election officials acting fraudulently. Of particular importance here is that the former is relatively rare (despite scaremongering by one side of the political spectrum) while the latter is decidedly less so.

Also, legally, neither one is treason.

I'm sure you read the article being discussed in this subthread (https://www.brennancenter.org/analysis/debunking-voter-fraud...) -- it makes the same distinction.

> (b) treason

I think you need to look up the legal definition of treason as specified by Article 3, Section 3 of the US Constitution.