because that niche is already pretty well filled with excel, and it's hard to unthrone a program that is actually really good at what it does.
On the other hand, casual users aren't a natural fit for excel. The program has way too many complicated features, and isn't as simple as it could be. This seems perfect for the casual user that just want to create simple stuff that looks good.
Excel is really, really bad at what it does. It won because of Microsoft's former monopolistic position. Lotus was much better. Most of the heaviest users of spreadsheets just build their own internal "corporate" spreadsheet program.
On the other hand, casual users aren't a natural fit for excel. The program has way too many complicated features, and isn't as simple as it could be. This seems perfect for the casual user that just want to create simple stuff that looks good.