ISS didn't have many countries as part of it especially chinaChina and indiaIndia IMO have many and varying reason.
China is somewhat capable on his own for going on venture like this having space station as you know Tiangong.
If you compare space program of chinaChina with nations participating in ISS excluding USA china, China has far more program running concurrently and joining ISS means chinaChina has to spend a considerable amount and resources for it's part in ISS. China is still a part of newly industrialize nations.
Also china is part of Asian Space Race.
China has to prove itself that it has been there as early as possible to compete with nations like indiaIndia and japanJapan.
Regarding indiaIndia there has been talks of letting participate isroISRO in ISS but in my opinion this is somewhat not possible because of lack of budget and technological barriers.
Isro didn't have any experience in projecting human in outer space neither have any crew sustainability module, although development is in undergoing for above points.
The annual running costs for the Japanese Experiment Module will be totally around $350-400 million (almost half of ISRO’s annual budget). Which means that if India has to participate meaningfully and do some interesting science, ISRO will need an almost 50% hike (to Rs.3,500 crore) in its budget. Although this is one-tenth of the cost of having our own manned space programmes, this is also the cost of having 50 Mars Orbiter Missions a year!
http://thewire.in/28441/will-isro-participate-in-the-international-space-station/
