Because that threatens the validity of randomization and the comparability of the counterfactual, if the article writer knows it'll be uploaded, they can do things like pick and choose which ones to work hard on. "Oh, I like this case, it accords with my personal politics, so I will work extra hard on it since I know it'll be uploaded." (One example from the RCT literature of how knowledge of the randomization before the intervention can be a bad thing: in one early study of steroids for babies, the hospital nurses 'knew' that steroids helped, and so they would pull out the randomizing ball and if it was 'wrong' for that baby, they'd put it back in and try again. This is why clinical trials try to use pregenerated randomness if they can't blind the nurses.) Even if mechanisms like that don't add a systematic bias, they do add noise and reduce the statistical power to detect an effect.

It also helps the ethics angle if you are simply holding back articles which you will upload eventually.