The oldest is the first, gets all the new stuff and in many cases remembers what it's like to be an only child. Then the second child, by accident or on purpose suddenly the parents no longer outnumbered the children.
Parents are already worn out from the first, strained financially perhaps and need to reorder the family once again. They get beat to shit but they love you and are so happy you are there. Luckily there is an older sibling to help with you from time to time.
Eventually the last child, maybe the third perhaps the sixth. the parents can stop planning for the unknown. The cloths and toys have been beaten to hell so they might as well buy something new.
Most importantly from a parents perspective it's their last chance to enjoy loving one of their children while they are young as best they can.
It really is just being a victim of circumstances. Every time you have a kid the money is different, the family makeup changes hell your parents dynamic changes. You aren't loved less just differently but make no mistake you are loved.
tl;dr Parents suck with middle children hence the Middle Child Syndrome, we're sorry and we love you.