A possible reason for the lack of a bronze "favoriting" badge is that favoriting seems to perform two somewhat different functions:
Users might want to watch a question closely for a time -- they're interested in possible answers to it, or perhaps plan to write an answer themselves if no one else comes up with a good one. Questions of this kind normally will be removed from a user's "Favorite" selection after a while.
Users might feel that a question and/or some of its existing answers are of special interest to them, sort of "canonical". Although the user's interest might wear away eventually, the inital intention at least is to add such questions to one's "Favorite" list for good.
I suppose that the existing "Favorite Question" and "Stellar Question" silver/gold badges only aim at favorites of the second type. And with 25 or even 100 "Favorite" marks, a question surely is of the "canonical" kind. Perhaps the SO developers felt that with 10 marks, one can't be sure if a question is only of passing interest to its followers, and therefore they didn't introduce a corresponding bronze badge. To find out whether the developers were right, one would have to delve into SO's data dump.
(Some evidence from tex.sx, mostly anecdotic: Ar the moment, 28 "Favorite Question" badges and zero "Stellar Question" badges have been earned [a few questions are close to "Stellar"]. The number of questions with at least 10 "Favorite" marks seems to be about five to ten times lower than the number of questions with a "Nice Question" badge, i.e., the former questions are probably of permanent interest to their followers. For tex.sx, I'd say: Yes, please introduce a "Worthy Question" bronze badge!)
EDIT: According to the latest data dump, the frequency of the proposed "Worthy Question" badge would be somewhat above that of the "Good Question" badge at SO. At tex.sx., "Worthy Questions" would be even rarer than "Good Questions".
Stellar Questions 419 0
Favorite Questions 3,694 28
Questions with >= 10 14,913 147
Great Questions* 986 3
Good Questions* 10,357 162
Nice Questions* 51,090 1,762
* as of 2012-02-05
EDIT: Another possible concern with regard to the suggested "Worthy Question" badge is that users are allowed to favorite their own questions, and so may chip in 10% of the "Worthy Question" requirement. I don't think this tips the scale against the proposed badge. (And if it's really a concern, the criteria for the [Worthy/]Favorite/Stellar Question badges could be changed to "Question favorited by 10/25/100 other users.)
Disclosure: If a "Worthy Question" badge would be introduced and retroactively applied to existing questions, I'd earn this badge five times at tex.sx.