The reason for this has been brought up before.

There is a maximum occurence value that is set to limit searches. The last someone checked and told me it was at 50000. That would mean that if a word appears more than 50000 times on the board, it will not be indexed. There is also a list of non-searchable words, regardless of the number of occurences. This list contains words such as "the" and "and" which have no reason to be searched for. Finally, there is a four-character minimum for words. This has caused controversy in the past, because many people want it reduced to three. The reason it hasn't has supposedly been for space concerns (it would add a lot more words to the database), but if new space has been purchased I don't know why this hasn't been changed.

So most likely the words "nobody" and "nothing" are either on the no-search list or have more than 50000 occurences.