They are available but usually at a higher price and that is why the M-Th times are used to get the lowest possible price. When you see a low price try other dates near that time including weekend dates and see what the price is. Prices are all about load factors and as seats are filled prices go up. There are usually more open seats on M-Th dates giving lower prices. Thanks for your comments.
Have a Great Day and Blessings to All!