TF2 has a huge amount of players but the overall community is fractured so badly that it may not be possible to bring Comp 6's to where we all want it to go. If you compare TF2 to CSGO you can see this divide more clearly.
CSGO players can be scrimming or pubbing and the vast majority are playing the exact same game mode with the same objectives as the Competitive scene. That translates to new players jumping into the CSGO comp scene with little change from the normal game they just installed and played. This can be said for other games like Overwatch as well.
TF2 players have a larger amount of game modes to choose from and there is no way to funnel the player base into the 6v6 comp game mode at this point. Players are not going to give up on payload and other Highlander type games so for the average TF2 player, there is no push or drive to enter the competitive scene like there is in CSGO.
In every previous Team Fortress generation, starting with original Quake, the goal of competitive play was always centered around CTF. The TF classes were created with the abilities to defend or capture a flag. This was always the format for competitive Team Fortress and that meant new players were automatically playing the competitive side of TF after they installed so any move to Competitive play was a simple and easy transition.
That simply did not happen with TF2 although CTF was played in the beginning. However, the old CTF maps were very poorly made (example: 2Fort or Well) and there was little or no effort to change them in order to fit TF2. That is when 6's took hold and never looked back to CTF, there were much better maps for 6's and it was a fresh new way to play the game.
Unfortunately, 6's is not what new players first experience when they install and play TF2 which is why I compared it to new players in CSGO and Overwatch. Considering TF2's age and fractured state, I really don't know how or if it is possible to get the player base we all want and need for 6's to really jump to for the forefront of the TF2 world.