Well, Enya's music is a little different. There are other modes and scales that are neither minor nor major that can sound sad. Likewise, major can be sad in the right context and style. With Enya, she keeps the tempo slow, the sound very roomy and choral and creates a more ethereal sound that people often feel reminiscent or ambivalent, which can be reduced in some as 'sad'... it can get very complicated lol. It's not like I know objectively what I'm talking about here