Why don't alkynes undergo preferential anti bromination to the degree that alkenes do? Alkenes, for the most part (unless there is a phenyl group or highly ionising solvent) undergo almost exclusively anti addition of bromine. However, under similar conditions alkynes, although undergo preferential anti-addition, it is not as exclusive as alkenes. Even with electron withdrawing substituents like two $\ce{COOH}$, only 70% is anti. (Peter Sykes, page 182)
Why?
From what I know of the mechanism, Addition of Br2 takes place in the following mechanism

For an alkyne, the intermediate would not be stable due to the ring strain on 3 member ring with a double bond in it, thus reducing the rate of reaction.

Mechanism referred from: https://en.wikipedia.org/wiki/Halogen_addition_reaction