A quick reply to Doppel: It isn't only familiarity. One very good reason for using ASCII is image contrast and ease of reading. Each individual letter has only two colors (foreground and background), is nice and thick, and in most cases is easy to distinguishable from other letters, especially when you start color-coding them.
Graphical tiles, while they provide more familiar icons, have to cram many more distinguishing characteristics into their space, and the resulting cramping strains the eyes to look at. It might be easy to confuse, say, a goblin with a halfling - similar size and shape, after all - and if you start trying to differentiate graphics between goblins, goblin archers, elite goblin hammerlords, and fire-breathing mutant death goblins, you might have a serious problem. Better graphics and bigger spaces for them ameliorate this problem a lot, but those require better artists and better interface code. Which costs money and even more importantly, time. In low-resource niches like Dwarf Fortress and most roguelikes, ASCII tiles are often a better design choice.
Graphics have a lot to recommend them - they are easier for the brain to translate, and the good ones do look a lot nicer - but "familiarity" is not the only reason to eschew them.