Casiquiare canal is the world's largest bifurcation. Since such a large bifurcation seems unlikely to be produced and even less likely to stay for centuries, I wonder how old it is and why all the water didn't just stay in Orinoco.
This is a part answer, as pinning down the actual date that the canal formed is not well framed in the literature.
According to the Texas A&M University web-chapter The Vaupes Arch and Casiquiare Canal Barriers and Passages (Winemiller and Willis, 2011), are quite vague about exactly pinning down the age of the canal, with
Extensive alluvial sedimentation and channel meandering provided subsequent opportunities for drainage capture between the Orinoco and Negro headwaters to the east of the Vaupes Arch, and at some point the Río Casiquiare formed a connection between the upper Orinoco and upper Negro rivers
and by using digital maps based on the topography suggest that
the upper Casiquiare course may have been captured quite recently by the Pasiba–Siapa–lower Casiquiare drainage network as a result of river meandering on the peneplain.
The article The Casiquiare river acts as a corridor between the Amazonas and Orinoco river basins: biogeographic analysis of the genus Cichla (Willis et al. 2001) are more direct in stating that
Unfortunately, the precise timing of the origin of the Casiquiare remains unknown
also stating that in studies of the DNA of species in either basin, there seems to be little dispersal between them through the canal. Specifically noting that the
Casiquiare River acts as a selective filter, where only some species are able to make the transition from stained, acidic water in the Negro River to unstained, neutral water in the upper Orinoco River.
Winemiller et al. (2008) state in the article Fish assemblages of the Casiquiare River, a corridor and zoogeographical filter for dispersal between the Orinoco and Amazon basins (2009), that
Hydrologically, the Casiquiare represents a stream capture in progress, but there is some debate concerning its formation
Secondly, an answer to your question as to why the water does not stay in the Orinoco, it is a question of elevation, according to Winemiller and Willis, (2011), it is a case of flowing downstream from the western edge of the Guiana Shield, the direction governed by the elevations of the two ends of the bifurcation.
At its origin at the bifurcation of the upper Orinoco, the Casiquiare is about 90 m wide and lies at an elevation of 120 meters above sea level (m-asl). At its mouth at the upper Rio Negro, the Casiquiare is over 500 m wide at an elevation of about 90 m-asl.
and passes through what Winemiller et al. (2008) describe as an alluvium filled floodplain, that coupled with the meandering and stream capture (mentioned above) has governed the course of the canal.
The Rio Negro being a major tributary of the Amazon River.
Lujan and Armbruster, authors of the chapter The Guiana Shield (2011) assert that
In the future, it is likely that a new drainage divide will form within the Orinoco downstream of the Tama-Tama bifurcation, and the current headwaters of the Orinoco will become entirely adopted by the Amazon