It failed because it wasn’t a true union between Egypt and Syria. The Syrian leaders at the time came to Nasser because they feared that their rule was seriously threatened by a well-organized communist group. So even at its inception, the motivations for wanting the union betrayed a certain asymmetry in power since the Syria feared an ouster. Not only that, Nasser enjoyed an immense popularity at the time of the UAR’s creation, creating an even larger power differential.

As was to be expected, Nasser became the union’s leader; Syrian political leaders were forced to go to Egypt - and essentially do nothing while Nasser ruled; and Egyptian personnel (both military and civilian) poured into Syria and replaced Syrian counterparts in the country’s administration. Those factors in addition to the disbandment of political parties in Syria and the introduction of land reform alienated significant portions of the elite. This eventually resulted in a military rebellion against the Egyptians which ended the incredibly short attempt at political unity between the two countries.

Cleveland, William L. and Martin Burton. “A History of the Modern Middle East.” Westview Press, 2016.