It's because they spent years and years only marketing the Fab 5 to infants and toddlers. Mickey Mouse Clubhouse was the only show Disney was producing involving the Fab 5 for YEARS. By the time a kid was 4 he/she thought Mickey was for babies because of this. They are only now starting to hold on to the kids as they age up. Hopefully they stick with it and make Mickey have longer lasting appeal.
This.
Plus Disney has ebbed and flowed with how to treat Mickey Mouse for generations. It even goes back to Donald Duck, who in the early years was easier to write for because Mickey had become synonymous with good and thus bland. Mickey was meant to be the everyman character, which like Charlie Brown, made him relatable but also somewhat bland compared to the characters around him. Yes, in the earliest shorts he was mischievous, but as his popularity soared and he became more ubiquitous, the company knew that he had to be the good guy all the time. They didn't quite know how to write stories for him, but side characters like Donald Duck could be the "mischievous" character... and thus easier to write for.
It got even more difficult to write for him as he became a corporate symbol. As the company's mascot, the weight of being "good" or "inoffensive" became even stronger, while at the same time the company continued to put his face on all number of children's product and yet selling his t-shirt at the parks.
Even in the 1950s Mickey Mouse Club he was a mascot more than any other type of character. And through the years the company has put energy behind other iterations of Mickey that have ebbed and flowed, including Mousercize, Disco Mickey, Vinylmation, Mickey and the Beanstalk, Mickey's version of A Christmas Carol, Runaway Brain, and of course park appearances.
So, for generations the company has not known how to market Mickey Mouse, with the main "problem" being his own stature in balance as "everyman," childhood appeal, and corporate symbol. Truthfully, it takes both courage and someone to care about the character to do anything more with him... someone like Walt.
