That's a pre-1900 POD that you got there.
The Central American state has reconstituted itself several times throughout history.
With backing by America and Mexico, the Central American states signed onto a multi-layer peace treaty, alliance system and trade agreement in 1907 known as the 'Central American Court of Justice.' The five nations involved though were all poor, and all had bad communication diplomatically and literally (it could took weeks to relay messages). With a Nicaraguan Canal though, you have a richer, more vibrant Nicaragua. This brings in foreign investment and capital, which Nicaragua could wisely invest into their local industry. Using the Court system, they could reinvigorate the Central American state that had been attempted several times already;
Federal Republic of Central America (Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua); 1823-1840, ending in Civil War.
Confederation of Central America (El Salvador, Guatemala, Honduras and Nicaragua); 1842-1844, ending in Civil War.
Federation of Central America (El Salvador, Honduras and Nicaragua); 1852 - lasted a month, ending in Civil War.
Greater Republic of Central America (El Salvador, Honduras and Nicaragua); 1896-1898, ending in Civil War.
As you can see, there's a general trend here
The biggest issues that kept tearing the Central Americans apart were liberal trade doctrine and slavery. The Conservatives were against the first and for the latter, and had the nominal backing of the Catholic Church. The Conservatives also had the backing of the military, which often overthrow the central and local governments. The Liberals were often disunited and squabbled amongst themselves. And this isn't even looking at the outside influence Mexico, America and the Brits provided
Despite the failure of a lasting political union, the concept of Central American reunification, though lacking enthusiasm from the leaders of the individual countries, rises from time to time. In 1856-1857 the region successfully established a military coalition to repel an invasion by U.S. adventurer William Walker. Today, all five nations fly flags that retain the old federal motif of two outer blue bands bounding an inner white stripe.
A Nicaraguan Canal, with the outside investment (and interference) it brings though, gives the Liberals the upper hand for once. Its highly likely in such a scenario that the Central American state is revived once again and has a fighting chance at lasting this time.