They’re probably the lowest bidder, so they keep winning contracts or meet certain requirements that others couldn’t meet for their product. As for NJT, they want their super-specific railcars, which adds additional cost because they’re the only buyers of them (and then there’s the issue of the Penn Tunnels which further restricts car specifications). Other agencies bought the same multi-level cars in the past but have since gone with other makers for future car acquisitions for reasons unknown.
Alstom isn’t only screwing up in the US either. Some quick searches yield similar delays and other issues in Europe as well. They need to get their act together.