The old building was 30 years old. Whether they replaced it when they did or a few years later, there is no way the taxpayers of NJ would have paid for the new building as they did the old building. The NFL puts an aggressive date certain on new stadiums being built and when they have to be paid for.
It's just a sign of the times. While NJSEA is the titular owner, the Giants and Jets secured private funding for construction.