Person of Interest was a network show that was always on the verge of being cancelled starring two character actors. It was also a procedural for the first part of it's run, which makes it harder to convince hard sci-fi fans to watch it.

Westworld was an HBO prestige drama with a huge, extremely well known cast and a sizable budget. For the first two seasons it was very popular, and it was heavily serialized similar to Lost.

I also prefer Person of Interest to Westworld, but it makes perfect sense to me that it's not as popular.