How can I communicate that the word limit prevents you from elaborating something in an essay?
How to communicate that word limit prevents you from elaborate something in essay?

You don’t. You take the time and put in the effort to make your argument in the available space. If you do that, more likely than not the resulting essay will be a better piece of writing that does a better job of putting your argument across than if it had been longer. Thus, you will have nothing to excuse or apologize for.
And don’t take it from me, many famous thinkers have expressed similar thoughts about the benefits of conciseness (and the difficulty of achieving those benefits). My favorite one of these quotes is Woodrow Wilson’s reply when he was asked how long he takes to write a speech:

“That depends on the length of the speech,” answered the President. “If it is a ten-minute speech it takes me all of two weeks to prepare it; if it is a half-hour speech it takes me a week; if I can talk as long as I want to it requires no preparation at all. I am ready now.”