Simple, really.
Transparent acrylic is transparent to visible light, like the 450nm wavelength blue, but it is not transparent to the long IR wavelength of the CO2 laser.
So when you focus the blue light from the diode on it, the light passes through, and does not heat the material, instead it will just continue through to whatever is behind the material.
When you focus the IR light from a CO2 laser on it, that’s a different matter, and it will heat/cut the material. This is also why clear acrylic panels can be used as a safety shield from the CO2 laser.
You will find that some colours are harder to cut as well. Things that are blue will be more difficult to cut with the diode laser. Blue objects reflect (rather than absorb) blue light, so the material will reflect away some of the laser’s energy rather than converting it to heat.