Who says so? One is either ignorant or ill-informed. (1) In Indian context, see this answer: answer to Do Indian Muslims hate People of other religion as Muslims of other part of world does? read more
Typically, when the question "why can't religions coexist" is asked, the focus is on the historic struggles among Christianity, Judaism, and Islam, although other religions are often included. Sometimes, a contrast is drawn between the pacifism of Eastern mysticism and the violence of traditional monotheism (Christianity, Judaism, Islam), though violence and extremism can also be found among the mystic religions. read more