Since menarche is rarely caused by ovulation (asked here 2 times, and they said its uncommon prior to menarche) , what is the main cause behind it?

Please see below: Menarche is a sign of uterus being prepared to conceive. If your uterus is not prepared, its not possible to get pregnant.

Maturing Hormones: You asked if ovulation went on for a long time before menarche. The questions you have asked are 'unique' and seem very much out of the norm and out of context. Why are you so invested in this question that you keep asking? It seems so suspicious from a physician's standpoint. Girls' ovaries mature. Ovulation begins and then menarche begins as hormone systems mature in the young female.

Pregnancy=ovulation: Menarche is marked by the beginning of menses for the first time. It is usually without ovulation [anovulation] due to immaturity of some glands at that age. As a result many girls do not have menstrual cramps initially. However, there are girls who ovulate from the beginning of menarche and therefore can get pregnant.

Confusing wording: Estrogen, the common female hormone triggers the buildup of tissue in the womb that will eventually break down and produce menses. When the body is ready, within a year,ovulation will eventually begin.Ovulation produces a secondary hormone that will also trigger menses on a more timely basis. Ovulation is never necessary for menses, but can make it more predictable.