Due to the presence of more s-character, the alkynes are more electronegative. Thus, in Ethyne, hydrogen atoms can be liberated as protons more easily.
The acidity of a terminal alkyne is due to the high level of s character in the sp hybrid orbital, which bonds with the s orbital of the hydrogen atom to form a single covalent bond. The high level of s character in an sp hybridized carbon causes the overlap region of the σ bond to shift much closer to the carbon atom. This polarizes the bond, causing the hydrogen atom to become slightly positive. This slight positive charge makes the hydrogen atom a weak proton, which a strong base can remove.
In the case of alkanes and alkenes, the s character in the hybridized carbon bonds is less, resulting in fewer electronegative carbon atoms and a corresponding lesser shift toward those atoms in the overlap region σ bond.