This is for safety reasons. Tackling is the only way of legally bringing down your opponent in rugby union. After each tackle the attacking team should be closer to their opposition's end of the field.
But there are certain laws on how to tackle and if these are not adhered to, penalties will follow. When you tackle an opponent, you cannot make contact above the shoulders.
I'm answering this from a rugby union perspective. When, where, why and how you make a rugby tackle is an important part of the game. If the ball carrier goes to ground, they must play the ball as soon as possible.  Of all the rugby league positions , second-row averages the most tackles. One of the most confusing aspects of rugby to beginners is what happens after a tackle. Know what tacklers and tackled players are and what you must do.
Rugby Tackle. It all depends. A 2012 New Zealand study found that over 659 tackles are made per game in professional rugby league. Understand this and you're well on your way to enjoying rugby.