This answer is not correct. Although the carbon atom to which the leaving group is attached looks sterically unhindered, it actually is blocked by the cis hydrogen atom of the vinyl group. Note the lack of room for attack in this space-filling drawing.

The sp² carbon-bromine bond is a bit stronger than a sp³ carbon-bromine bond and this also will increase the energy of activation to the point that the competitive E2 mechanism will be favored rather than substitution.