STO Well positioned, page-225

Because 1.5 degrees is where climate science tells us that we still have a chance to keep the planets climate systems intact.
Anything beyond 1.5 degrees and all bets are off, because of feedback loops in the climate system like the one in the article below.
Another feedback loop is that shrinking sea ice at the poles, means there is less white surface to reflect the suns heat back, and which is instead absorbed by the dark blue ocean. This heats the ocean up more, which in turn melts the ice quicker.

Or were you not aware of these effects and why 1.5 was so crucial?

Arctic methane deposits 'starting to release', scientists say | Climate science | The Guardian

Runaway greenhouse effect - Wikipedia