Conjunction scope may benefit from parallelism and higher-level structure, which is easily accessible when joining two matching non-terminals in a context-free grammar, but much harder to determine in the local views of graph-based dependency parsers.
with edge-based scoring, the object is not accessible when scoring where the preposition should attach, and PP-attachment is known to depend on the object of the preposition (Hindle and Rooth, 1993).