Maybe you weren’t the first choice for the project. Or the second. Or the third.
None of that matters. That’s just ego. Get over it.
Assuming you love the project and the people behind it, all that matters is you’re a part of it now. You get to do meaningful work. Be grateful for the opportunity. Give everything you got. Put your whole heart and soul into it. Be generous and kind towards everyone you encounter.
And go kick some ass.