In my experience, widowhood suffers from the fact that there are no generally accepted social rules any longer.
So it is easier to stop inviting us widows, or not start again, as most of us could not accept invitations during the last months of our husbands' lives.
Nor do they know "what to say" so they do not offer condolences or ask if we need help just after our bereavement, and not having done so, find it impossible later to offer help.
Much the same thing applies to anyone who is single.
My strategy is to invite people myself, thus signaling that I would like to see them, and will be happy to accept an invitation.
This is not entirely satisfactory, as you "cannot" invite those with families at Christmas, New year, Easter etc. but on an ordinary day of the week, it should be possible.