Not only do some races go up and down in single years. In other cases, a race that has been historically strong will suddenly be down for a year or two for no apparent reason. Also, at least around here, most races have results online but the only thing you can find for some are last year's results or even just last year's winning time, which can be deceptive at times, and some don't have any online presence other than being listed in a race calendar or two.
True, if you are really searching out a race with top level competition, you should be able to find it by looking for the bigger races that have a proven history of fast winning times and deep fields. However, if you're just looking for a tune-up race, various factors may keep you doing whatever is available on a given day and hoping for the best. Of course, if you do so, you should understand the risk associated.