D&D has a ling history of throwing subraces at a wall and seeing which random features stick.
Looking at the two stickiest subraces D&D have produced - Drow and Tieflings I think we can see some commonality.
1 - Visually distinct. Drow by their colouring and Tieflings by their horns.
2 - They both tap into the angtsy outsider who is more badass than the normies theme.
3 - They are both, nominally at least, evil inclined. This allows players to be good guys rejecting their heritage, neutrals struggling with their heritage or evil embracing their heritage.
4 - They are both D&D original creations. I know evil elves and devil spawn aren’t new ideas, but the way they are represented in D&D is distinct.


If you want to generalize a bit further from there, some rules for creating a ‘sticky’ subrace could be:
1- be visually different and appealing.
2- be an outsider. People who play D&D know they’re not mainstream, so have an affinity for outsiders.
3- have some built in roleplaying hooks, especially as it relates to core rules (good/evil axis)
4- be original, not a reskin of someone else’s IP.


The Kender are an example of a non-sticky subrace (treating them as a subrace of gnomes). I think most of it comes down to their chaotic nature which rubs many other players up the wrong way as a lot of Kender players turn up the chaos to 11.