I think stewards can be more involved with financial aspect of regions. How about giving them an option to steal gold from tax treasury of their region? This will make lords consider choosing their stewards more carefully. Also, having a steward should have some benefits. If you look at some big realms, the number of lords is almost equal to the number of nobles in the realm. Knights of cities almost always get more gold than rural knights. It is very hard for rural lords to attract knights when they can't provide any other incentive.
Any idea?
Hmm, I believe you are looking for incentive for being a Steward, for rural knights. Currently Steward controlling the food warehouse, not gold tax income.
I used to promote my characters in elections, by saying my characters have experience of being Stewards of this X and Y regions. May Ketchum Steward of Sir Temple, Sirion region at one point in her past. Being Stewards, meaning you are trusted by the Region Lord. How about we make it, that Steward has a greater claim on region vacancy?