The main reason, of course, that Ho Chi Minh wanted to establish an independent country was the fact that he wanted his country to be free and independent.  Vietnamese in general were a strongly independent people who had been controlled by outsiders for centuries.  When they started to see the chance to gain independence, they were eager to take it.

After that, independence came about largely because of outside circumstances.  Ho and the Vietminh were able to have the chance to declare independence because of World War II.  When WWII came along, the French were ejected from Vietnam.  Later, the Japanese were ejected as well.  This left a power vacuum that allowed Ho to be able to declare independence.

We can also say that Ho was allowed to declare independence by geopolitics.  By the time that North Vietnam actually became independent in 1954, he had a great deal of support from the Soviet Union and from China.  The fact that he and they were both communist caused them to support his push for independence. 

So, we can say that Ho declared independence because Vietnamese were eager for independence and because of WWII and the Cold War.