Question

why is the IDL considered with respect to the 180 degree meridian only

Answer 1

This question may need rephrasing, but if I understand it correctly, you want to know why the International Date Line goes through the 180° meridian.

Well, it doesn’t. It zig-zags around a bit. But the important thing to understand is that the International Date Line doesn’t really exist. It’s just a convenient way to explain why the time zones to the east are behind UTC (round 12 hours), while the time zones to the west are ahead of UTC (also by about 12 hours). The result is that the time on either side is roughly the same, but the date to the east is a day behind the date to the west.