I am curious if there is any inherent quality of monotheistic faiths that have lead to their dominance in the modern era over older belieg systems like Greek, Celtic and Teutonic paganism of old. Did it provide something that these other faiths simply could not supply, or does it come down to the aggressiveness of proselytization?
One of the most respected and well-known arguments for this was made in 1994 by Fowden, who said that in late antiquity (which sees the rise of Christianity and Islam) monotheistic religions were better suited for culturally/philosophically uniting diverse populations, and therefore some of the most successful political/militaristic movements growing amid the middle East's increasing economic and cultural integration happened to be grounded in monotheistic ideas.
However, the rise of the major religions is of course an incredibly complex subject that is still being studied and debated, and will be for the foreseeable future. This means that there is no single consensus on it and that there can be no single simple answer. There are always the diverse elements of economics, politics, non-religious cultural trends.
The paths to the political and economic dominance of communities loyal to Christianity and Islam were in fact incredibly different. so, to simplify it to a simple answer is to ignore many factors.
Plus, once they had power, maintaining power is a totally different beast. Both Christian and Islamic empires permitted, and sometimes even supported (financially, militarily, theologically), the worship of saints, which often was a way to allow people to maintain their traditional polytheistic beliefs. So, to that extent, these religious communities were polytheistic.
Also, you have to consider that we are talking almost 2000 years of history and that the monotheistic empires had periods of immense strength and significant weakness, and at different periods there were clearly many economic, ecological, and technological factors playing significant roles in a region's ascendance--such as the 9th and 10th c salinazation of the Sawad (Baghdad--Islamic empire kingdom) or the rise in wool trade and the enclosure movement in 14th through 16th c Western Europe, which helped bring unprecedented wealth and a desire for profits from rents and interests, forcing a greater willingness to turn to "natural law" (based on Greek--not Christian--philosophy).