This is because the mechanics of the current map generation.
Under the existing map generation rules, the spawning of "resources" are semi-exclusive, they are generally not likely to cluster with each other. For instance, if a given region has a lot of Forests, then resources such as Horse and Iron will be very unlikely to spawn in this particular region, since 1. Horse and Iron cannot generate in Forests 2. the resources are coded to be not spawning too close to each other.
Ley Lines is labelled as "resources" in the code. This means that most of Ley Lines will spawn in Snow, Tundra, or Desert, since there are very few existing resources spawn in these regions to compete with Ley Lines' spawning rate. The player's complaint about Ley Lines are not entirely because they are really rare, but because they are rare in normal regions but can be abundant in the Desert or Snow. If just "increase" the number of Ley Lines, it will only result in more and more Ley Lines spawning in the worthless snowy regions.
By limiting the spawn locations, Ley Lines are "forced" to be spawn in no-Snow region, therefore making them more accessible to players.