Why are rewards of unpopular adventures not changed?
Rebalancing is a very sensitive issue. Unless it is done right, it could negatively impact the in-game economy and value. There has to be an up-to-date framework and ruleset, which will guarantee the value of rewards inside each adventure. One way of maintaining the value of a reward is making it unique by limiting the sources from which it can be acquired. For instance, "The availability of Magic Beans is related to Fairytale adventures" is such a rule.
Over the years, The Settlers Online has evolved and changed. You now have more uniquely skilled generals and the wide variety of buffs provide more opportunities, altering the initial difficulty of adventures. As a result there might be a need for the difficulty to be adjusted. This is just one of several variables that need to be taken into account. Other basic rules have to be re-evaluated. The rule "Adventures should be doable with default generals" is one of those, and it is a very important one. It has a big impact on the amount of losses during an adventure and it also impacts the balancing of the rewards. This is one of the main reasons why some players are able to complete adventures and get a better reward/loss return. The discussion about the Platinum Ore rewards in the Arabian Nights Adventures is a very good example. A simple increase of rewards won't do the trick.
There are no plans to adjust the balancing of adventure rewards in the near future, since this would require a lot of preparation time.
Have there been drop rate changes for adventure and treasure searches?
As mentioned above, re-balancing is a very sensitive topic. That is why there have been no changes on the drop rates of adventure and treasure searches for quite a long time.