Why does it seem so hard to get a chinese visa?

I am a business woman from south south region of Nigeria. I was at Abuja last week to obtain one, so I could attain a trade show in Shenzhen, next week. I believe I had all the necessary requirements for a business trip - my incoperation certificate, 6 months current bank statement, hotel and flight reservations, my company ID,letter of invitation from the organizers of the trade show and letter introducing my company to the Chinese embassy. But to my greatest surprise I was told at the security post that I need a letter from the Chinese government, inviting me to China, and that an individual or an organization does not have the power to invite a foreigner and the authorization from the Chinese government have to arrive at least one month in advance and in original copy and not a scanned copy, otherwise it is almost impossible to get one to attend the trade show next week, but contrary enough, that if I can spend N250,000.00, then it could be possible to obtain one to meet up with the timing. I like to know why this is so. This is really surprising to me. Also, I would like to know where the Chinese Consular Office is, in Cross River State. Thank you.