ERP systems are very necessary in an organization, are also open source, and their community is great. Odoo has a very stable core, powerful and fast to learn. Another reason is modularity, which makes it really reusable and easy to make tricks and improve functionality. Odoo is growing fast and growing unstoppably.
Many reasons, most important, they focused on its success at customers. Such as
1) Open source
2) Low total cost of ownership
3) Easy to learn
4) Easy to customize
5) Web based
6) All-in-one solution
7) Provide standard application
They made it as open source, so developers can easy reach to it and review its code and contribute actively in its success by performing success customizations and success implementations.
There are much more, you have to try then you know really how it's great articles.