Why does bicyclobutane have considerable ring strain?

Bicyclobutane possesses considerable ring strain primarily due to significant angle strain. In organic chemistry, ring strain arises when bond angles are forced to deviate from the ideal tetrahedral bond angle of 109.5°.1

Bicyclobutane consists of two fused cyclopropane rings. As seen in cyclopropane, the geometry of a three-membered ring requires C–C–C angles of 60°, which is a substantial departure from the preferred tetrahedral geometry. This compression of bond angles creates intense angle strain. Furthermore, such small, highly constrained ring systems are often destabilized by torsional strain, which results from the eclipsing of bonds on adjacent atoms. While larger cycloalkanes can adopt nonplanar, "puckered" conformations to minimize these energetic penalties, the rigid, fused structure of bicyclobutane prevents such relief, maintaining high levels of strain that influence its chemical reactivity.1