I suspect it was always a festival of light...One
of them anyway.
Depending on which pagan or craft practitioner you speak to, the festival of light could also be Imbloc, which is traditionally celebrated on midwinter's day and marks the return of spring. The Christian feastday of Candlemas (Feb 2 usually) was conveniently located next to it in the same manner Christmas was relocated close to the solstice - for purely practical rather than scriptural reasons.
When reprogramming a society's spiritual belief system, it's generally wiser to co-opt and meet the older tradition half way, rather than try to engineer a complete and immediate overhaul.