He did not "hide". As already pointed in comments, the "dark cave" is where they entered, not the previous state.
It is explained [perhaps Nefesh haHaim?...] that, before eating the fruit, they did know the good and evil, but it was like something 'outside'; and by eating, they 'mixed' the evil into themselves.
Before, there was also tenptation (for example, the Snake) but it was like the idea to put one's hand into fire: it is possible, but everybody knows that it is bad. Now, we often do not know what is the evil -- it is blended in ourselves. And this is called דעת , to mix knowledge. But this is a damage.
[And, it is known (see in comments on the question) that, maybe the fruit would be permitted if they waited until Shabbos. So, it was just a matter of time. Now consider that, you do not explain to your 4-years old son all the details on how to make a baby: he has time to learn, why disturbing him now... You nor "hide" him where to buy drugs or a gun... Just better to him not to know.]