One could question why her god, if it exists and is so wonderful, didn't look upon her more kindly and heal her?
There's more than one kind of healing.
Every time such a thing happens in the New Testament, it's not to show some healing magic, but to illustrate a point.
Even Paul, in his later life, was losing his sight. From the tantalising clues in Acts and a few of his own letters, he may ave been almost blind.
If Christ had a purpose for Paul in his disability, then he can equally use us in ours.
By the way, Joni was a cultural Christian before her accident.
She only accepted Christ AFTER it.
Her sequal to "Joni" - which charts her accident, the depression which followed it, and her coming to Christ, entitled "A step further" is very revealing, showing just how valuable and fruitful her life could be, and would become.