Illustrated Glossary of Organic Chemistry

Orbital electronegativity: As the s character of a hybrid orbital increases, so does the apparent electronegativity of the atom which hosts this hybrid orbital.

Carbon is sp3 (25% s character)
pKa ~ 50

Carbon is sp2 (33% s character)
pKa = 40

Carbon is sp (50% s character)
pKa = 25
Acidity of an X-H bond increases as the electronegativity of X increases. The pKa trend in methane, ethylene, and acetylene is explained by orbital electronegativity.