Assuming you mean nicknaming limitations, it's to prevent the player from calling their Pokemon something obscene.
Not only is this not in the spirit of the game but trading Pokemon with offensive names is obviously something undesirable for the game creators to have happen, especially when a large part of Pokemon players are children.
Since trading happens worldwide, the censor takes into consideration a list of words from various languages, not just your own. Because of this, it may try to block a name that looks safe to you.
Of course, the censor isn't perfect. Safe names may be incorrectly blocked and some obscene names might slip by and be allowed. Previously, "Cofagrigus" was blocked from trade even though it was the Pokemon species' name.