One of the axioms for the choice of cards in a Highlander deck is that each card is permitted only once, except basic lands and cards that override format imposed restrictions, e.g. Relentless Rats. The determining criterion is the English card name.
The deck must consist of a minimum of 100 cards.
Essentially all printed, tournament legal Magic cards produced by Wizards of the Coast can be used in deck construction. The following exceptions and rules must be adhered to:
All new Magic expansions become legal on the worldwide release date of the edition, as in all sanctioned DCI formats.
A Sideboard is not permitted.
The Highlander format takes, because of its inherent slower nature, often longer than other regular constructed formats. The 50 or 60 minute time allotment per round is often too short when the longer search and shuffle effects are taken into account. Allowing a sideboard would largely exceed the time limits.
In addition, it is a part of the excitement of deckbuilding to tune the deck for the expected metagame. It is a part of the decision making process which card answers are required for challenges the opponent could present. A card which is deadly against a certain type of deck, may prove all but useless against other decks. Games after sideboarding would be more dependent on the luck of drawing the first hoser card.