Why is it difficult to get an invitation letter from Chinese Company?

Am a Caterer and the name of my company is La Royale Events, recently I upgraded my business to premium level and I need to upgrade my equipment as well,I contacted some Companies in China requesting for their product,and when I made my intention of coming to see thing and make the purchase personaly they seem to develop cold feet when I requested that they should send me an invitation letter to make me process my visa to China, but all this companies are yet to respond to my request of invitation letter,I contacted Shenzhen Tonistar co.Ltd in Guangzhou also CHAOZHOU fENGXI SHENGS HOU PORCELAIN ART FACTORY,up till now they are yet to respond to my request.Does it mean the Company in China are not ready to give invitation letter.When they refuse to give and that is the condition to get visa,should i forget about China and look elsewhere for my purchase?Or what do you suggest I should do?Also I want to go for this Canton fair in Guangzhou,I join this their Buyer E-Service Tool,when I requested for an invitation I was directed that I should contact the company I have the intention to patronise to give me an invitation letter,honestly speaking am very tired and this kind of things is highly frustrating,and it can scare away a serious buyer.Thanks