Santoku's have quickly become the popular first knife for cooking enthusiasts and chefs. Previously the longer and heavier 'chef's knife' was preferred, mainly for its versatility and durability.
Here are the three main reasons people are leaning more towards the Santoku:
The Santoku is a relatively modern invention, taking its shape from a range of older knives; keeping the best elements of each and refining it into a new all-purpose kitchen knife.
The broader body of the knife is designed like a Usuba or cleaver where the uniform width throughout lends a greater balance to the blade. The snub nose tip also allows the knife to still be nimble on smaller tasks, where a cleaver/nakiri/usuba would be too difficult to manoeuvre.
Historically, professional kitchens have influenced the home kitchen in knife shape and length. In recent years things have shifted somewhat. More and more, knife makers are creating knives with the home kitchen in mind as people are cooking more adventurous items and cuisines.
Enter the Santoku.
The Santoku length is suited to home kitchens where bench space are limited and chopping boards are smaller. The shorter length allows it to move around the board with ease, whether cutting on an angle or rock-chopping ingredients across the board. No other knife is better suited.
Santoku's have a somewhat peculiar shape; more curved than a Nakiri, flatter than a Gyuto and broader than a petty. The reason for this is threefold. 1) The up-curved tip allows you to do roll cuts, guiding the knife back and forth, 2) The flatter belly is great for vegetable preparation, where the the blade edge comes into full contact with the board after every stroke, and 3) the broader tip brings the balancing point further up the blade, giving you greater control on smaller tasks.