This carbon just lost its bond. Now it's positive, electron-starved, and desperate.
A primary carbocation has just one carbon neighbor. Its plus charge stays crowded on one atom.
Neighboring C–H bonds lean in. Their electrons spill into the empty p orbital. That's hyperconjugation.
Now give it three methyl groups. Nine C–H bonds can feed it, and alkyl groups push electrons inductively too.
So tertiary beats primary: more alkyl groups spread the positive charge, so it's more stable.