Brief answer: It is all about wavelength.
Detailed answer: The key question is how much of the energy will be absorbed by the material you want to cut. The light on the material could either be reflected, transmitted or absorbed.
The 450nm light of our laser diode is absorbed very well on organic materials. Those 40W CO2 tube lasers have a wavelength of 10600nm, only a small fraction of their light is absorbed while most of it is reflected.