To stay on topic as a general rule with regards to any form of information or skepticism.... "fact" is something an majority have agreed upon and hence from that day they call it an fact.... we call the grass green because we reached an consensus on the usage and interpretation of both the words "grass" and the word "green". Coming back to your question of  "why"... it's simple.... It's harder to defend than to attack facts in general and especially on global warming where the data is interpretable in many ways to the highest bidder for a mouthpiece P.H.D. degree for both organisations/goverments/companies etc at both sides whether it's a bio-hippy or the oil-junky. Long story short....It's a fight between who is better in convincing and the "facts" are secondary as long the objective is reached...which is convince you of the existence or myth of global warming.