Why a straight line and not a circle for tessering?

I know, of course, that L'Engle's novels are fiction. But I have always heard that time is a circle that doubles back on itself. It makes sense to me. If time is a straight line, then it has a beginning and and end...even if "wrinkling" or "tessering" makes time travel quicker. But it does not make sense that time has a beginning and an end. I guess it also doesn't make sense that I am asking this question when the writing is fiction! Just curious to know if anyone else has thought about this!