I don't believe you'll find a definitive answer to this, as there are too many factors that may have impacted the Grand Coulee Dam's national prominence. However, looking at a few different reasons, we may be able to come to a reasonable conclusion. To start, I will point out that Grand Coulee's low prominence isn't for lack of trying. Every night since 1989, the Grand Coulee Dam lights up with a half-hour long laser light show, which is one of only two such permanent large-scale laser light shows I'm aware of in the US (the other being at the Crazy Horse monument in South Dakota). They also have a large and interesting museum and visitor center at the base of the dam, and the dam is even referenced in several Woodie Guthrie songs, after the singer visited the area for a few weeks.

To start, we'll look at location, which may be one of the more important factors in the Hoover Dam's enduring popularity. The Grand Coulee Dam is roughly 85 miles from the nearest major population center, Spokane, as opposed to the Hoover Dam's 34 miles. A day trip leaving from Las Vegas takes about 30-40 minutes, depending on traffic, whereas a trip to the Grand Coulee Dam is 2 hours each way. To actually get to each dam is a rather large contrast as well, as the road to the Hoover Dam skirts settled areas, but never strays far. Getting to the Grand Coulee Dam is a far more solitary trip, meandering down Route 2 through mostly farmland and unsettled areas. Neither of these factors by themselves would be enough to tip the scales, but it's hard to deny the impact the longer and lonelier trip may have had at keeping interest in the Grand Coulee Dam down.

Another factor to look at would likely be the workforce used to construct each dam. To sustain tourism levels, there needs to exist a feedback loop in which people awed by the grandeur of each dam return to their homes and tell others about their trip, inspiring others to do the same. While it wouldn't guarantee a later crowd draw, it would certainly help if this loop existed even before the completion of each respective dam. However, the Grand Coulee Dam was built by a mostly local workforce, drawn from the surrounding Douglas, Grant, Lincoln and Okanogan counties. While this would generate plenty of regional interest, the Hoover Dam was built in an area without a major population to draw on, initially. Las Vegas, a city of 5,000 people, soon saw an influx of up to 20,000 workers, many of whom would keep in contact with their families about what they were building. After each project drew to a close, most workers on the Grand Coulee Dam returned to their families, likely due to the fact that they had never gone far from home in the first place. The Hoover Dam temporary worker camps, however, turned into a more permanent settlement in the form of Boulder City.

Neither of the above advantages likely tell the whole story, however. There are far more subjective issues to consider as well. The Hoover Dam is a relatively unique design, being a very tall and imposing gravity arch dam. The Grand Coulee Dam is impressive, but somehow less overwhelming in scale. Its height of 550' as compared to Hoover Dam's 720' doesn't sound like much of a difference, but confined within the walls of Black Canyon, Hoover Dam's height appears to be in a class of its own. The total length of the Grand Coulee at 5,200' isn't quite as exaggerated, due to the flatter geography of the surrounding area, as well as the bend in the geometry of the dam after an addition started in 1967. Another factor to be considered is the length of construction and the timing of the completion of each: Hoover Dam was completed in 1936, taking just 5 years, while Grand Coulee Dam didn't become operational until 1941, nearly 8 years after the cofferdams were initially built. By this time, the US had shifted focus to the upcoming war that was enveloping the world.