There's probably different causes at different times. Some of these effects that are the target of replication tests are relatively older, when people were less aware of some of this phenomena. To be fair, some of this stuff is unintuitive: for example, some of the major journals would require internal replications, over several samples, the rationale being that if someone shows an effect in 5 samples with slightly different designs, it's probably "real." It's not like some of these things were just based on single samples (although some of them certainly were). Of course, now people are aware that 5 small samples does not large-sample replication make.
My intuition is that another cause is that academics is flooded with researchers collecting a lot of data, under a lot of pressure to produce positive findings to attract money. Hype, TEDtalks, grants, hypercompetition, and so forth. Academics now is horribly incentivized to be popular and bring in money (from peers, it's important to note), rather than to be correct. Add in a complex subject, like human behavior, and it's a recipe for disaster. Academics is also full of conventions, that attain the strength of power structures; when these are baked in it's hard to change them because you have people who attained power under those conventions in charge (nothing conspiratorial really, just people have their biases and blindspots).