Only a small fraction of users on Strava even put their bikes in, much less label them as a specific type of bike.  That is all pretty recent.  Many of those don’t bother to remember to change their activity to the correct bike when they are done, so you would have a ton of error just in that factor.  Add in that the lines between what defines a “gravel” bike and a “mountain” bike are really blurred now, it would be difficult, if not impossible, to come up with a criteria that everyone would understand and follow when entering their bikes.  I know a number of people who use the same bike for gravel and mountain, just swap out the wheel sets.  

On top of all of this, you have over a decade of historical data that did not have that designation, so that would confuse the data.  Anything you look at now that is just a “gravel” leaderboard would only be a small percentage of those who actually rode or ride the segment on their gravel bikes.  Same for if you looked at a “mountain bike” specific leaderboard.  The difference between this idea and the split of EMTB and MTB is that EMTB was never combined with regular MTB.  Prior to Ebikes having their own activities and segments, they were not allowed on Strava at all, so while there were some people breaking the rules, it was a very small number.  It was easy to make a clean leaderboard for Ebikes that was separate because it was a whole new activity that was not allowed on Strava previously and could start fresh.  Trying to split up the biking segments into sub-sets at this point would just leave you with a ton of error filled leaderboards that carry no real value.