Yeah, that's a good question. It isn't consistent. The OBJECT pdf has an example without using the commas, from page 11, which suggests it wasn't required, at least not at first:
Any number of messages can be sent in one fell swoop:
(ego
x:50
y:50
setMotion: MoveTo 100 100
setCycle: Reverse self
)
Using a comma feels impure to me. It distracts from the smalltalk/Objective-C keyword message syntax. Maybe the comma was optional. Maybe some developers preferred using it. It doesn't seem to make sense as a requirement, unless there were a possibility of encountering a colon in a place where it doesn't follow a selector name.