Central America actually was united for a few years. First under Mexico, then the the Federal Republic of Central America.

When the regions you mentioned above declared independence (excluding Mexico and Belize) they were annexed by the Mexican empire in 1821. Within 2 years the Mexican empire started to fall apart and the Republic became independent.

The Republic would survive a short 17 years before it fell apart due to infighting. The country was split between the conservatives and liberals politically. The conservatives believed in limited voting rights, a strong central government, and the authority of the catholic church. The liberals believed in wider voting rights, the separation of church and state, and states rights.

The government at the time, led by the liberal Jose Manuel Arce, managed to alienate both sides. The government was already in deadlock, but his actions led to a military coup against him. He eroded much of his liberal support by making deals with the conservative party. He also attempted to remove and replace the governor of Guatemala. This led to open rebellion by both conservative and liberal groups, and he resigned in favor of his conservative vp.

Liberals under Francisco Morazan managed to temporarily stabilize the country, until 1837. By then, the government was weakened and the populace disillusioned by years of fighting, as well as a cholera outbreak which some saw as divine punishment. Rafael Carrera lead peasant revolt, and the conservatives seized the opportunity and launched their own revolt. In 1838 the country began to dissolve into its constituent states, and Morazan fled the country in 1840, effectively ending any hope for a unified Central America.

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TL;DR Central America was unable to effectively unify because of the political divides among its people, leading to the collapse of the country into its constituent provinces.