Yeats - Most influential poet?

Okay, this might be a bit of a stupid question but I'm genuinely curious about this. Why is Yeats considered to be the most influential poet of the 20th century? I've come across this statement in some literary criticism books and on websites. But none of them explain why this is so. They seem to assume that this is something everyone knows (or should know) about. I've asked this on other forums and got some vague statements about how he was an important Symbolist poet and the role he played in the Irish Literary Revival. I wonder if anyone could give me a more specific answer? :)

Apologies if this has been asked before and for any mistakes in my English. It is not my mother tongue. :D

He was very good poet but I am not sure whether he is so influential.

Yeats was indeed a very good poet, but as already suggested, I don't know that he is considered the "most influential" poet of the 20th century. Others that one might consider of equal if not greater influence might include T.S. Eliot, Wallace Stevens, Ranier Maria Rilke, Federico Garcia Lorca, Pablo Neruda, and even Ezra Pound.

He is a transitional figure for English poetry, who bridges the gap between the late 19th century and the Modernists, and "The Second Coming" is generally recognized as the first major Modernist poem in English.

Yes, I thought that he was not very influential as well. But I saw that statement made by multiple books and websites and it confused me. Anyway, thanks to everyone who replied. :)