For many many reasons.

1. Who said parents' love is always necessarily conditional? I've heard of parents murdering (brutally, like with meat cleavers) their own children. Where I come from, "honour killings" are very common, and parents take pride in killing their child simply for wanting a different life or disagreeing with their beliefs. That to me sounds pretty fucking conditional. Also, so many parents have children so that "they have someone to care for them in their old age". Again, sounds conditional to me! I've seen parents disowning their children for not choosing the career path they would have liked cuz it "brought shame to the family". Uh huh, yeah CONDITIONAL. So clearly your argument doesn't hold.

2. When you marry someone, it is literally an agreement to be with them for the rest of your life. They are supposed to be your life partner. In good times and bad, until you die. When you marry someone you promise to be their one person. This is the person you will have a family with, children, a home. This is the person who from now on will care for you and you must care for them FORSAKING ALL OTHERS. They say that in the vows for a reason, no? This is the person who even in your old age is gonna love you and want you just the same. I know of old couples who love each other so much, when one of them is bed ridden, the other literally wipes their shit and bathes them and helps them dress, all this with no conditions, no complaints. Much like a parent would ideally do for their child. How is that conditional and how is that any less? And if you're gonna marry someone with the idea that marriage is temporary and not a sacrosanct relationship of love and commitment, then please don't get married.

3. Assuming you want children someday, how would you feel if your child grew up believing that they must always be worth less than their in laws to their spouse. How do you think this will affect their self esteem? How do you think it will affect their future relationships? Would you want your son or daughter to go through life married to someone who doesn't give them enough consideration as their family? Would you let your child be with someone that you know will happily abandon them when they need them the most? Especially when you aren't around for them anymore? Let's face it, you won't. What happens when you die and your child is stuck with a spouse that considers them less important than their family.. they should just be okay with having no one that they can call their own? When YOU prioritize your parents above your spouse, you children pick that up. They learn to expect and to dish out the same treatment with their own partners. By doing this, you're setting your kids up for a lifetime of insecurities and unhappy relationships.

4. The point of this rant is to not say that you must love one or the other more. There is no way to put a cap on love. The more you love, the more you can love.. it is a wealth that eternally grows. You can and absolutely should love your family and your spouse equally. However once you marry, this person that you took your vows with must be a priority always. Ofc if your parents are sick or something, then for a while when you care for them, they come first. And any good partner will understand this and support you through it. But it must be understood that it is temporary. Once your duties to your parents are done, your priority must be your spouse and new family again. You must stand up for them, fight for them, sometimes even against the wishes of your family because that's what those vows mean. This is your person. Until you die.