Why WASD?

At some point in the late 90's or early 2000's, first-person shooter games on the PC started by default mapping the movement keys to the W, A, S and D keys of the keyboard (the cursor keys being the more common default mapping in the infancy of first-person shooters, ie. the early 90's.) Some other game genres, such as third-person shooters, quickly followed suit.

This was indeed much more convenient than the cursor keys, especially since games were becoming more and more complex by the year, requiring more and more keys to control. In most standard PC keyboards the cursor keys are kind of isolated from other keys, and thus the user needed to reach farther to use any other such keys. However, the WASD keys are not only immediately surrounded by plenty of other keys, additionally (when the WASD keys are used with the index, middle and ring finger) the thumb ends up conveniently resting on the spacebar, thus bringing that key conveniently into the mix as well. Also the pinky finger comfortably rests on the ctrl and shift keys.

Very typically the most common default key mapping for most games, besides the WASD keys themselves, have been for quite a long time for the spacebar to jump, E as the primary "use" or "interact" key, R typically to reload, and F for some sort of secondary "use" key. Some games that require even more keys may by default use the Q, the G, and even the Z, X and C keys. (While not universal, the C key is very often mapped by default to crouching, although in some games the left ctrl key is used for that.)

However, some game will need even more keys than those, and may thus extend to use keys that are much farther away, such as the J, I or even the M keys.

But one has to ask, why did WASD in particular become the almost universal standard? If you notice the key on the left of A is Caps Lock. Some games do actually map it to some functionality, but rarely. In fact, many games leave it to its default system functionality. There might be some technical difficulties sometimes getting the Caps Lock key to remap to something else.

Wouldn't it have been better to make the ESDF keys the default for movement? In other words, shift everything one slot to the right. This way the A key would have been conveniently on the left of the S key for some functionality. Even the Q key is now relatively easy to reach as well, and would thus act as an additional convenient key.

(Of course there's nothing stopping a player from remapping the keys like this, as the vast majority of games offer the option to do that. But WASD has become ubiquitous because all games use it by default, and the vast majority of people won't bother to change it.)

Some games use some more awkwardly-positioned keys by default, like the Alt key. This could well be remapped to the A or the Q key instead, for instance.