"The presentation is not over."
Well, it isn't over until after it has finished.
Hence my translation of "is over" (present) with "finished" (past).
Your "was not over" would probably be "had not finished" (pluperfect: bitmemişti).
And "bitmiyor" would be "is not finishing (now)" which would also imply that it's not over now, just as "bitmeyecek" (will not finish [in the future]) also implies that it's not over now, perhaps.
You could also think of "over" as a past participle, like "broken", which is an adjective but also the past participle of "break".
If the bottle is broken (now) then someone broke it (in the past). And if the bottle is not broken (now) then that means nobody broke it (in the past).
Even if they were right in the middle of breaking it (now), it still wouldn't be "broken" until after they have finished doing so, any more than somebody tugging at the handle of a window (he is opening it) means that the window is, at that point, open.
Because "it is over" means more or less the same thing as "it has finished".
The finishing is in the past, and the result (being over) is in the present.
The Turkish here (bitmedi) is something like "it did not finish" or "it has not finished", which is why it is not over now - because it did not finish in the past.
If it were finishing right now, it would not be over yet -- it would only be over after the finishing is finished.
"was over" indicates a state of being over in the past... in which case the finishing would have to have happened even earlier. If the party was over yesterday at 11 o'clock, then the finishing took place before 11 o'clock.
I imagine that would then be bitmemişti "had not finished = was not over".
Compare other action/state pairs such as "has died = is dead" or "someone has opened the door = the door is open".