Why do people say don't judge others when he who is spiritual judges ALL things?

It is not the whole Biblical quote  "Do not judge others lest yea be judged"   This would be more along the lines "Do not judge others because the same sort of judgement might be applied to you".

       This is sort of similar to saying that the one without sin should throw the first stone.

Because people often enjoy twisting scripture to justify their wicked heart.

Everybody judges others, whether they will admit it or not. 

I always admire when a Christian tells me not to judge, then, informs me I am going to hell for being a non believer. 

Religious people realize that judgement is part of life. It's how we make decisions and protect ourselves. Those that say don't judge have little understanding of the purpose of judgement.

The judge not mantra is invariably used by those who wish to excuse their sin.

1 Corinthians 4:5  King James Version

5 Therefore judge nothing before the time, until the Lord come, who both will bring to light the hidden things of darkness, and will make manifest the counsels of the hearts: and then shall every man have praise of God.

When it comes to Judging people we only have at best 1/3 of the equation. Where God has the complete knowledge, what is in the mind, in the heart as well as the actions.

As a general rule 'don't judge' is a pretty good piece of advice - you don't always have all of the information you need to make a snap judgement about someone - and as the old saying goes 'until you've walked a mile in their shoes'.

On the other hand, a blanket statement of 'don't judge' doesn't always apply, because it is possible to take a look at someone's words/actions and make an informed judgement - but you better make sure you've covered your bases, otherwise that's where the 'lest ye be judged yourself' part kicks in.

As far as some supernatural deity being the 'supreme judge', that has yet to be proven.

God is the judge of ALL things. We rarely have all the relevant facts about a person or an event to arrive at a sound judgment. Besides, most matters going on around us do not involve us. We can have an opinion, but in this day and age, everybody's opinion is viewed as being correct for them, which means opinions are worthless. But in spiritual matters, there has to be judgment, regarding what is to be believed and practiced, and in a congregation of believers, God has an arrangement for keeping a loving balance.

When most people say, "Don't judge others!" it's because they are unspiritual and don't want to be judged themselves. But a spiritual person knows he or she is judged by God, and when their conscience is clear before God, they don't care what anybody else says about them.

If you are human with a brain, you judge naturally. People don't read the Bible, they only say what they heard someone else say. 

Matthew 7

1 “Do not judge, or you too will be judged. 2 For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you."