The format standards of notebooks have changed and perhaps you were using an older standard that Github used to handle better. Gtihub may have deprecated that one when they upgraded how they handle previewing notebooks.
(Is it possible you had Jupytext in your environment and don’t now? That will render notebooks as .py form and keep them in sync.)
The use of nbviewer just requires sharing a different URL as your notebooks would still be on Github. Sometimes it is overkill/somewhat redundant as the Gtihub render has improved much recently. However, you’ll get better results especially with long or wide notebook and those using Plotly and other javascript enhanced rendering.