I think the default graphics might be part of the reason this game is as well-known as it is (as mentioned by Toady at Dwarf Moot).
I don't think most new players are going to be comfortable playing with ASCII initially, even if they'd like to be able to. I don't see any need for Bay 12 Games to include graphics with the game, though. If someone learned of Dwarf Fortress from a friend, that friend can tell them to use the Lazy Newb Pack for graphics. If someone learns about the game from the internet, there's a good chance they'll watch a YouTube tutorial. Many of them are going to be using graphics packs, especially those targeting new players. When searching for a Dwarf Fortress tutorial on YouTube, a new player is sure to see several examples with graphics, and those YouTubers using graphics in their newbie tutorials will probably mention the Lazy Newb Pack. The community is providing for the game pretty well with all these tools, graphics, and tutorials.
What graphics pack do you think would be a good candidate for inclusion? A lot of the existing graphics packs would be kind of tricky for Toady to legally include in the game with so many artists who have contributed to them. GemSet and CLA might be good candidates since (I believe) they were both made by a single author, but both those packs are pretty ASCII-ish and may not be the best choice for a player uncomfortable with ASCII.