The answer is because the calculation of the calendar for the church year is very complicated. By only doing Sundays, Solemnities, and Feastdays the task is simplified tremendously. There is only one celebration for Sundays, Solemnities, and Feastdays while there can be up to four possible and optional celebrations for some of the weekdays during the church year. Doing the calculations for the weekdays is possible and it is something we are planning for future versions of the Faithware Librarian.