As a complement to the answers already given, you should keep in mind that the numbers for these badges seem to be determined uniformly for the entire Stackexchange-2.0 network, of which the representative site is stackoverflow.com. That site has traffic several orders of magnitude greater than this one, which makes the badge dynamics totally different. Questions fall off the front page in a few minutes, but there are way more voters and some questions and answers have vote totals in the thousands. Thus, "Notable Question" is strangely difficult to get there, since your question is hard to get noticed, but "Epic" has a lot more of a chance, whereas here it is virtually impossible (only a few of the top users have it, compared with the enormously common "Good Answer"/"Good Question" badges, or "Necromancer"). And "Tenacious", which on Stack Overflow is a dime a dozen, is pretty much out of reach on this site, even though it's only silver.
Note that "Notable Question" is not so easy to get here, because of the lower traffic. I wouldn't say that is one of the more unbalanced badges, whereas Epic and especially Legendary clearly are.