Making the town hall level the main determining factor would discourage many people from ever upgrading to the next town hall level.
Any clans that allowed people at new town hall levels to be in their clan would be penalized. It would be in the best interest of all clans that war to kick out anyone that decides to upgrade to the next town hall level.
I don't see how this would be a positive addition to the game.
I don't think it would discourage upgrading TH at all actually. Acquiring new troops and defenses as well as being able to progress is incentive enough. also some people upgrade their TH just for the extra camp space and spell, but don't upgrade any defenses. This doesn't add to their weight. This is not a healthy war mechanic in my opinion.
Okay, so let's look at a scenario in which matchmaking would be based on the town hall levels as the main determining factor instead of defensive strength.
Let's say my clan has 5 TH10s, 5 TH9s, 5 TH8s, and 5 TH7s, all completely maxed on both offense and defense for their town hall levels.
And your clan has the same distribution of town hall levels, except that they are all brand new to those town hall levels (the TH10s haven't built their infernos, the TH9s haven't built their x-bows, etc.).
But our clans are matched in a war because we were the only two clans that were searching for a war at that moment in time with that exact mix of town hall levels.
Is that a fair match?
Is it more fair than if we had a different mix of town hall levels, but our overall strength was similar?
Because it makes it harder on the matchmaking system (and thus indirectly cause mismatches).
It's much simpler to make a matchmaking system based on the assumption that every base is evenly upgraded and discourage uneven upgrading, than it is to make system which will work for both, evenly and unevenly upgraded bases.
I doubt it was ever the intention of SC for players to hold back upgrades just for the purpose of getting more favorable war matches.