Why is Krishna invisible?
Krishna is the most social being. He's the source of all personalities and of personality itself. At the same time, Krishna values His privacy. He prefers the company of those who prefer His company. He also respects the wishes of those who don't want relationships with Him. He doesn't force Himself on anyone.
His energies are so vast that He can create, maintain, and annihilate unlimited universes by His will, without needing to be personally present. Likewise, heads of state often make decisions affecting the lives of millions of people who never see them personally.
Prison inmates certainly don't routinely get to see high-ranking government officials. In a similar way, all of us in the material world at some point in the past decided to reject God's company and along with it the rules of conduct befitting Krishna's devotees. As a result, we've been offered residence in this temporary, material universe, which is similar to a prison.
This world is made as a replica of the spiritual world, with one key difference—Krishna is absent. Krishna is omnipresent by His energy and by His presence in every atom as Paramatma, but as far as we're concerned, He's invisible, unless He chooses to reveal Himself to us by spiritual revelation.
Check this out:
Bhagavad-gita, 7.25, Purport:" it is said that the Lord is covered by the curtain of yoga-maya and thus ordinary people cannot understand Him."