As it currently stands it only shows kills since they are the most prevalent stat looked at by players. For the most part people want to know how many kills they have and how many the highest person has either in the day or by the hour (I know we also include all time and monthly).
If people want to know the top assists, killstreaks, etc... They can always feel free to use /stats top [stat they wish to check] and it will display it in their chat. I think the way it's setup right now in spawn would have to be changed completely to accommodate including every single one of these stats, which doesn't seem necessary. I will say that people do look a lot at who has the highest killstreak, so potentially adding that isn't necessarily a bad idea.
I don't think it's a bad idea overall, I just don't see the need of it. I feel like it wouldn't hurt to add top killstreaks, but it also wouldn't change much either since you can just use the command to check.