Because Ball Pythons like to have quiet, dark, secluded hiding places and the 1/2 log does not provide that so he is seeking it out elsewhere.

If you can see him then he can see you and will not feel as secure.

If you provide him with a solid all the way around hide with a small entrance he will probably prefer that. The 1/2 rock ones available commercially work well but a solid color plastic bowl with a door cut out will work just as well.

It may also be heat related as that is a fairly small enclosure and the light seems bright. How are his basking temps and is there a thermostat controlling that light?