So here's one possible explanation that is going to be heavy on information that isn't necessarily canon:
One of the dangers of traveling the galaxy is of course, as Han says in IV, charting courses through hyperspace, so that you don't fly through a star, bounce too close to a supernova, etc. If we assume the endpoint in the Unknown Regions (which I believe has been said), then that area is particularly not well mapped out when it comes to hyperspace lanes, and they might not have been able to identify the segment BB-8 has anyway. So you need the beginning of the map to just know how to get to the last piece. The map in its entirety is a safe route through the Unknown Regions, theoretically.
Or, the more likely explanation, "it is this way because it is this way. Just sit back and enjoy it" All Star Wars movies have those moments.