How and why are all the substituents in methyl cyclopropane eclipsed?
The geometry of the molecule is highly restricted by the presence of the ring.



The presence of the ring caused the $\ce{C-C-C}$ ring to be planar, because every triangle is planar.

Since there is only one carbon atom directly to the right to the circle, it must be eclipsed, so are the hydrogen atoms by VSEPR.