The field is over-crowded, for one. Two, its participant pool is almost exclusively undergraduate students on university campuses in WEIRD (western, educated, industrialized, rich, and democratic) nations. Three, experimental designs may rely on questionably reliable, possibly retrospective self-report to infer patterns of affect and cognition. Four, experiments are often conducted in highly contrived laboratory environments. (In-situ data collection as people go about their lives is becoming more common with the rise of smartphones and wearables, however.)

As in most domains of science, researchers are also pressured to publish regardless of the quality of the work, and replicated findings are usually not considered interesting or valuable enough to publish.

This is anecdata, so take it with a grain of salt. (I minored in psychology and worked as a research assistant with a social/cognitive psych lab as an undergrad.) Also, there is a ton of extremely good science in psychology. It's a shame the recent profusion of low-quality work obscures it.