Whenever you kill a monster that may drop an item, a lot of randomness is played out. No need to go into detail just here (if you really want to know, you can look it up in the spoilers), suffice it to say that depth is the single most important factor. You may, of course, find rather valuable and/or useful stuff even on the very first level, but the chances are rather small.
In short: if you want to find better things, you need to go deeper.