Why does a stroke primarily affect speech?

Not true: Not true that strokes primarily affect speech. Symptoms of stroke include speech changes, weakness/paralysis, decreased sensation, confusion, memory problems, vision changes, etc. The most visible symptoms of stroke are muscle weakness which may or may not include speech - because they are fed by the middle cerebral artery which feeds most of the cerebrum. However, you can have "silent" strokes.

Location: It really depends on location. Left-sided strokes tend to cause speech problems since speech centers are located in the left brain in most of right-handed people.