I think the answer to that is because if you were DSDs mum your DP would be living with his dd full time and so leaving her for a few hours wouldn't be such an upheaval for him. As it is, every moment he spends with her feels like a blessing, as he has to spend half the time missing her.
I have my DCs 6 days a week and for me, a night off is very welcome, but when they're occasionally away for 2 or 3 nights, especially if I'm not doing anything fun to take my mind off it, I do miss them.
50/50 sounds like an ideal scenario, but actually in think it's hard, a bit like working PT - you get the best of both worlds, but also the worst of both worlds.
I know exactly how you feel about being second best. My DP has 2 DDs 50/50 and I constantly have to remind myself that it's not a competition, that of course in some ways he will put them first, but in others I know he will take a night off from them when I have a free night so that we can spend time together.
It sounds like you won't truly believe that you are a priority (not above his DDs, but up there with them) until something happens to prove it. For me it was after a week away on business, when DP popped in to see me before going to see his DDs. I encouraged him to go and see them sooner rather than later, in the hope that he would come back to see me once they were in bed.
He said there wa a 75% chance he'd be back! but they might want him to stay with them. I accepted this (although I said I thought it was more like 5% he'd be back). But when I let it go and accepted that I'd see him the next day, I felt ok about it.
If I had a newborn with him, I know I'd expect more from him though. It's tough for you and I hope he steps up and shows you how important you and your dd are to him.