To explain away the things they didn't understand.
And for whatever reason even when we started to understand things we didn't give up the crutch.
I think humankind has always had a sense of the Divine based on the reality that there is no civilization ever known to exist who did not acknowledge some form of deity. And indeed, many of the legends, lores, myths, allegories, symbolic concepts, etc. have arisen out of a desire to explain that deity or deities.
But while I suppose some people might use their religion as a 'crutch' to avoid consideration of other possibilities or truths, I do not think most people use religion as a crutch. I think for most it is a strong desire to understand, explain, or make contact with something in which they are intensely curious or in which they strongly believe.
Our understanding and perceptions of deity have changed over the millenia just as our understanding and perceptions of medicine, technology, the environment, the Earth, the universe have changed over the millenia. Why should the religious be accused of using religion as a crutch when the same accusation isn't made toward those researching medicine or the environment or the universe?