Why is Shlita (שליט"א) only used for Rabbi’s and not everyone? Why not wish everyone to live a long life?

Shalom, Thank you for your question. You are absolutely correct that we wish everyone a long and healthy life. The appellation "Shlita" is generally applied exclusively for rabbis only because of common usage – there is no reason other than as a sign of respect (just as I as a boy called all my teachers "Sir"). But in fact it could well apply to everyone – and Rabbi Avigdor Neventsal Shlita, the rabbi of the old city of Jerusalem, uses the term after everybody's name, be they rabbi or not. (Though in practice you might make people feel uncomfortable if you started writing "shlita" after everybody's names, because they will think that you think they are more important than they actually are or think they are). Blessings - May you live for many good days, amen – Shlita!