Why were Nixon’s policies towards China considered more successful than those of his predecessors?

I think that you are asking why Nixon's policies towards
China were considered more successful than those of his predecessors.  I have changed
your question to reflect this.

Pres. Richard Nixon's
relations with China are considered more successful than those of previous presidents
because it was Nixon who opened diplomatic relations with the People's Republic of China
(aka Communist China) for the first time.  Before Nixon, presidents had lacked the
ability and/or the desire to open relations with
China.

Nixon believed that relations with China would help
the US.  He believed that opening relations would put pressure on the USSR to be less
hostile to the US.  He also believed that opening relations would help the US get out of
the Vietnam War more easily.

Because of this, Nixon worked
for some years to get China to be receptive to the idea of relations with the US.  Nixon
was clearly the driving force behind the establishment of relations between the two
countries.  For this reason, his policies are seen as more successful than those of
previous presidents.