It's because the elder no longer has freedom of speach regarding certain issues. How may some in the congregation feel if he counselled them from the platform about raising children in the truth? Or, if his son was now known to be a drinker or used drugs recreationally while still living at home, could the elder counsel the brothers on these matters?
The thing is, as long as what he says is from the Bible then there isn't a problem with what he says. However, some in the congregation may feel that the counsel is a little hypocrtical since members of his family have not been a good example when it comes to avoiding that kind of conduct. The result is that the impact of God's word could be lessened because some people may, without realising, be put off the message because of his family situation.
If the elder is humble, and it sounds like he is since he still serves as an unbaptized publisher, then he will know that this is for the benefit of the congregation and will not look out for his own position and interests.
Jehovah's Witnesses have often been accused of twisting the scriptures, but cases like this show how serious they are when it comes to living by the Bible even though it is not always convenient to do so and may, in fact, result in great sacrifice.
It is important that the congregation knows who is disfellowshipped since such ones may have a tendancy to draw others away with them. Maybe not intentionally, but even just through their conduct and attitude.
I hope this helped even in a small way.
All the best.
Contrary to an answer below, disfellowshipping is scriptural. When a man in Corinth was unrepentantly immoral, Paul told the congregation: “Quit mixing in company with anyone called a brother that is a fornicator or a greedy person or an idolater or a reviler or a drunkard or an extortioner, not even eating with such a man.” (1 Corinthians 5:11-13) The same was to occur with apostates, such as Hymenaeus: “As for a man that promotes a sect, reject him after a first and a second admonition; knowing that such a man has been turned out of the way and is sinning.” (Titus 3:10, 11; 1 Timothy 1:19, 20) All Christian faiths should disfellowship unrepentant wrongdoers.
It is also scriptural for an elder to step down when one of his children does not remain faithful. 1 Timothy 3 provides the qualifications for elders and ministerial servants. Verses 4 and 5 say that they should be "a man presiding over his own household in a fine manner, having children in subjection with all seriousness; (if indeed any man does not know how to preside over his own household, how will he take care of God’s congregation?)" Once the circumstances have been taken into account (each case will be different) the elder may be removed or chose to step down of his own accord.