Here is a slightly different suggestion; it was a problem of travel times. As has been established in many other answers, the location is within the Unknown Regions. There is no reason to suppose that we cannot pinpoint exactly where in said Unknown Regions, using the identifiable clusters, however, we do not have enough information of this region to chart a hyperspace route there.
Say we wanted to get there sans hyper-drive, how long would it take? The entire Galaxy is 100,000 light years in diameter, and just going by eye we might guess that the Unknown Regions are about a third of this. Let's assume that we only have to travel half way into them, we need to cover approximately 17,000 light years. So even if we travel very close the the speed of light for the entire journey, it will take a long time. Although the traveller may experience time dilation (no idea if this happens in SW) Luke would not and so would be dead before anyone arrived to fetch them.
So we really need a hyper-drive for this. Here is an extract about said hyper-drive from the (now non-cannon, but still roughly accurate) west end games RPG rule-book;
To get technical, hyperspace is coterminous to normal space. That is
each point in real space is associated with a point in hyperspace, and
adjacent points in real space are adjacent in hyperspace. If you head
north in hyperspace you travel north in real space. Real object have
a "hyperspace shadow". That is there is a star or star-like object in
hyperspace at the "same" location as it occupies in real space. This
is a danger.
Collision with a hyperspace shadow can "instantly vaporise" an object in hyperspace. So that's why we need nav computers;
All but the smallest hyperdrive-capable starships are equipped with
nav computers. Nav computers hold a wealth of statistical and map data
charting the positions of stars, rouge planets, known asteroid fields
and other dangers.
Only well charted regions are available on nav computers;
Going to a star system that has never been visited before is very
dangerous. None of the obstacles along the route have been charted.
Only the crazy (or desperate) would attempt to travel straight to an
unexplored star system; usually, explorers travel a short
distance through hyperspace, drop into real space to take readings,
travel a short distance again, and so on. This is time consuming, but
While the map may have given enough detail to physically identify the region, and it even looked like it had a partial hyperspace route (that red line thing), a whole hyperspace route would be require to get to Luke. Only when paired with the rest of the map was a whole hyperspace route, or enough information to plot one, available.