Because only changes to these buildings are currently tested. When and if other buildings will also get a rebalancing depends on the test results here.
All buildings should have gotten this rebalance to be tested. To only re-balance one type of building will leave others to be overpowered and misused. I have yet to hear of any guild using traps but I am sure they are since they probably will be making it hard for the others to take sectors.
This not only puts the bigger players at a halt to their farming activities but an even greater halt to the smaller players to grow as now they are limited to a small number of fights per day (which translates to a small amount of rewards daily) which they will find GBg boring and avoid it, thus hurting guilds that often depend on them.
So, why don't I propose this since you (Inno) want to change this mechanic:
Make it that attrition resets twice daily. You read that right. One at server reset time of midnight. The other at noon server time. This way, those who are engaging in GBg (including the small players) can have peace of mind to get more fights in to get the rewards. You can still have your fix of 66.6 for the attrition prevention as I am sure attrition will still rise even with this in effect.
In practice, a player starts with 0 attrition. They get 20 fights in before attrition rose up to 20. At noon, their attrition resets to 0. They get another 35 fights in for the same attrition of 20 due to a sector having SCs capped at 66.6%. They are out for the count until server reset.
Another player that is medium starts with 0 attrition. They get close to 100 fights for 45 attrition with a few SCs placed. At noon, their attrition resets to 0. They get another 40 fights in until they call it a day with close to 40 attrition. They, too, are out for the count until server reset.
At the end of the day, the players get around 55 and 140 fights respectfully as well whatever rewards they need to grow in their cities.
The reason I had said every 12 hours for attrition reset is because some may take advantage of being there both times while others might have enough time for just one. You cannot punish the ones who are dedicated to GBg and know how to play it well, whereas you cannot punish the medium to weaker players just because the larger players tend to get everything very easy within their reach.
This "one size fits all" methodology and mindset has to go.