From the documentation:
If an explicit threshold value is given, Binarize ignores the Method option. »
so either use
Manipulate[Binarize[yourImage, binOnly], {binOnly, 0, 1}]
or
Manipulate[
Binarize[yourImage,
Method -> method], {method, {"Cluster", "Entropy", "Mean",
"MinimumError"}}]