YellowBear,
You asked the question "Any idea why". OK, proof, no, ideas yes. You may not like what I have to say, but, yes, I do.
First let me start off by saying that I am glad that they did not put Walleye back in Sprague, at least not yet. Walleye are very effective feeders that can and
will clean out prey fish if there is not an over abundance, or
if there is not sufficient cover to protect them. Let me give you some examples.
In Southern Idaho, on the Nevada boarder, there is an impoundment called
Salmon Falls Creek Reservoir. When I grew up I fished that reservoir a lot, because I could. It had been planted with LMB and SMB and crappie and perch and silvers and kokes and ....... What was apparent was that the trout were always struggling to find food, the Bass were almost non-existent, the crappie were few as hen's teeth. On the other hand, the Squawfish (yes, that is what the official name was in those days) and perch were so common it was disgusting. Perch were all 3 to 4 inches long and the Squawfish often swam and spawned in schools that numbered in the thousands. Sometime in the late 70's the State of Idaho started to plant Walleye fry. There were so many squawfish and so many perch that no walleye showed up for years. Finally the Walleye population built to the point that they started to spawn and best of all eat perch and squawfish. Those walleye went to town.
Even today, SMB are now plentiful, perch are few but large, squawfish just almost don't exist, crappie are back in large schools and the trout grow big and fast. The walleye population has crashed now that the excess food is gone, but they did their job.
In South East Idaho, Oneida Reservoir is on the Bear River, a few miles upstream of Utah. Again, trash fish was the name of the game. Massive carp populations, stunted massive populations of perch, not much more existed. The state put Walleye in there and they cleaned the perch right out. After time, the carp started to be reduced as well, but Walleye are not a great treatment for carp problems due to their smaller mouth. In this case, Oneida was not as large as Salmon Falls and the Walleye did not leave any room for anything else. Still, suddenly the river below had trout in it that was not there before.
In summary, walleye can clean out a lake if it is not large enough and if it does not have enough food in it. Of course, if the Walleye cannot spawn, then this is annon-issue. With Sprague, if walleye were planted now, with so little food already in there for the Bass, the walleye would ruin the lake. Now, in the future, if trash or bait fish get overpopulated, then Walleye would be a good choice. The same is true for the Tiger Muskies. Maybe later, but now is too early.
Now, to answer your question, two factors; one is Eastern Washington State University and two is the Spokane Indian tribe. Just do an internet search and you will find that those two groups have teamed up a lot in the last 10+ years. Walleye have been the target of a lot of those investigations.
Chris Donley appears to be complicit at least, but most likely a partner in this. I believe that his actions, as well as the conclusions made by the State's own research team, seem to suggest this. Many of us have discussed this on this site, as well as other sites, even in person, but how do you ignore the fact that if you read the studies yourself you see very little to suggest that Walleye have done any harm. If anything, the walleye do harm to the "Northern Pike Minnow", saving far more trout and metalheads and salmon fry then they eat. The inconsistent statements the State have published regarding Walleye would suggest that either they are fools or they think we are fools. I will leave that to you do determine.
Let me offer a couple of quick examples. The tribe and State did a kokanee study on Roosevelt and discovered the limiting factor was the fact that the water temperatures did not thermocline. The fish had to move up and down too far each day to feed and still get back to cool enough water. The same thing with the trout. So, how do they respond, they gut the limits this year and open up the main spawning arm during the spawning season. Second, the state published in their walleye guides several times that Walleye did not spawn in Sprague. They seemed to protect the sizes, push the fish larger and larger while protecting the large ones by only allowing only one to be taken. Then they blame an overpopulation of Walleye, of large size, as the reason Sprague was not fishing well. Then when they get challenged on this site about a claim that the Walleye were hard to catch because they had too much food, they suddenly get their private parts exposed. Tell me please, how does a fish that "does not spawn in Sprague", a fish that has the large fish protected, a fish that they admit their own calculations in their study were flawed, suddenly overpopulate, and still has so much food that they are hard to catch?
So, why no walleye in Sprague, I propose the following reasons, in order:
1) too little food right now to support them.
2) the Spokane Indian Tribe does not want them.
3) the State of Washington "powers that be" don't like Walleye.
4) in today's era of political and environmental correctness, Walleye are an invasive species and therefore need to be destroyed.