why does Elizabeth think Charlotte chose the dark, less attractive room to sit in?

" Elizabeth at first had rather wondered that Charlotte should not prefer the dining parlour for common use; it was a better sized room, and had a pleasanter aspect; but she soon saw that her friend had an excellent reason for what she did, for Mr. Collins would undoubtedly have been much less in his own apartment, had they sat in one equally lively; and she gave Charlotte credit for the arrangement."