It's a very old, very primitive program. It predates cursor keys, printers that could use proportional fonts and a lot of other things we take for granted today. Its handling for things like printer drivers was astonishingly primitive.

For decades, this was enough: it was the best-known word processor in the world. So the company got complacent and did not improve it much.

Then WordPerfect came along, which had superb printer and font handling, and was much richer in a lot of ways. WordStar dithered while WordPerfect took over: there were several new, incompatible WordStar apps, and if you had to learn a new UI anyway, why not switch to the leading competitor?

Then the market switched to Windows and largely rendered WordPerfect irrelevant, too.