Why does the Orthodox church allow divorce and more than one marriage?

Orthodox canon law (and therefore Orthodox Churches) do not allow or recognize divorce as such.

 What actually happens is that in some cases of divorce that has taken place (in spite of every effort by the Church to prevent it), a party may after some time approach the bishop and seek permission to obtain a penitential second marriage under specific circumstances:

(1)  The innocent party who is the victim of adultery, according to Matthew 19:9 in plain translation

(2) The innocent party who is the victim of another absolute end of the marriage for other extraordinary reasons 

(3)  A party who has tried to save the marriage but at the end of the process is able to convince the hierarch (bishop) that a second marriage would be a safeguard toward salvation.

Remarriage is permitted up to three times in Orthodoxy. Orthodox Christianity recognized that the unthinkable may sometimes happen, and that the best approach is not to deny that there was a marriage in the first place, but rather in some cases to recognize that a second penitential marriage with reconciliation to the Church is better than the alternative.

Source: http://www.orthodoxanswers.org/why-does-the-orthodox-church-allow-divorce-and-more-than-one-marriage-do-annulments-exist-in-orthodox-canon-law/