Because samaritans believed that Jesus said:

`Believe me,woman,the hour is coming

when you will worship the Father

neither on this mountain nor in Jerusalem.

You worship what you do not know;

we worship what we do know:

for salvation comes from the Jews.

But the hour will come----in fact it is here already-----

when true worshippers will worship the Father in spirit and truth:

that is the kind of worshipper

the Father wants.

God is spirit,

and those who worship

must worship in spirit and truth.

 The woman said to him,`I know that Messiah----that is,Christ----is coming;and when he comes he will tell us everything`Jesus said `I who am speaking to you,``I am he.` 

Jesus stayed in Samaria for two days,and when he spoke to them many more came to believe.Then they believed Jesus to be the Savior not only of Jews or Samaritans,but rather of the world.