Well, for several reasons, but in England none of them are legal. Some other places in the world, however, have naming laws. The justification for these laws is sometimes social, sometimes grammatical, and sometimes practical, religious or political. Portugal has forty-one pages of banned names (which include all shortened forms of names as well as nicknames).
Kind Italy, full of pity at the thought of a poor child saddled with a stupid name, bans ridiculous and shameful names, and a similar feeling is presumably behind Sweden's decision to ban the names Metallica and Ikea. Germany insists that a child's name should not lead to humiliation (Germany also bans non-sex-specific names, surnames-as-given-names, and proper nouns). In Mexico the same sort of principle of avoiding humiliation has led to a ban on literary characters and film franchises (so presumably James Bond is a double no-no (a no-no-no-no?)).
Grammatical reasons limit Icelandic names. You can only have a name that can be written in the Icelandic alphabet (so no Cs, for instance) and it has to be convertible into Icelandic genitive form. Danish names must follow standard Danish spelling (so you can't have a Hannns, for example).
There's a ban on Hebrew spellings in Morocco, and Tajikstan is thinking about a ban on Muslim names. Saudi Arabia bans names such as Nabi (prophet) and Jibreel (Gabriel) which are seen as contrary to religion. Saudi Arabia also bans foreign names such as Maya and Linda, a presumably political principle shared with Kyrgystan, which is proposing a ban on Russian ones.
From a practical point of view, California bans diacritical marks (ie what are usually called accents, as in José) because they're a nuisance on computers. Other states of the USA ban pictograms and numerals for the same reason, and New Zealand bans names which are unreasonably long. China also bans names which can't be entered into a computer, which apparently, astonishingly, has meant that one Chinese couple were unable to call their child @.
Lastly, Malaysia bans the names of animals, fruit and vegetables.
Oh dear...and I have so wished for a Malaysian grandchild called Warthog Banana Turnip, too.
Word To Use Today: a name that's banned in some other country. Like mine, perhaps (presumably banned in Portugal because it's a shortened form of Sarah).