Because it is very very faint. When you look at the eclipse, the atmosphere of the Earth is receiving no illumination from the Sun, and you can see the faint solar corona. But when you look at the Sun at other times with a sun filter, the sunlight falls on the Earth's atmosphere and illuminates it so brightly that it completely hides the corona. When you use the filter, you are reducing the light from the Sun by thousands of times, and you are also reducing the light from the atmosphere and the corona by the same amount. To see what this does, look at the Sun during a total eclipse WITH a sun filter. This will make the corona go away completely.