We had an incredibly fun time with our first Swords & Wizardry session and are wondering why non-human character races have a cap for maximum level.

For instance, in the core rules:

Dwarven player characters must be Fighters or Fighter-Thieves. Multi-classed Fighter-Thieves are limited to 6th level as Fighters, and may not advance beyond this point. (For more information on multi-classed Characters, see below.) A Dwarf who is purely a Fighter may advance beyond 6th level only if the warrior has Strength of 17 (maximum 7th level) or 18 (maximum 8th level).

Could anyone enlighten me on the reasons for these caps?