Updated: Sep 23, 2019
Because he's not perfectly tired.
Yes that seems super simplified- but most truths in parenting are pretty simple! I've had many clients tell me that their baby/toddler will fight bedtime for TWO hours! Every Night! A quick evaluation shows us that baby is simply not tired at the time they want to be bedtime, but 2 hours later! So we have to work on moving bedtime closer to a more suitable time for babies (between 7-8 in most cases.)
Another time I had an exhausted client tell me that ever since daylight savings time baby kept fighting bedtime for a whole hour. The reason for this was that baby was still on the old schedule, and did not shift on her own, so we had to help her do so! (If you're curious about how to prepare baby ahead of time for DST, check out this post ... even though the dates are for last year's DST). We have to remember that babies are little humans. We can't fall asleep just because someone tells us to (ok I definitely could at this point in my life as a baby and toddler mama haha!). Babies will not go to sleep unless they are TIRED. Not undertired or overtired, but actually tired. Many newborn moms get so frustrated because they know a good bedtime for babies is between 7-8pm (truth) and their little babe will just holler and not sleep until midnight. This is because newborns actually go to bed on the later side (between 9-11pm) for the first months of their lives! And if we ever try to get a baby to sleep when they aren't tired yet, well they're not going to like it! And when a baby doesn't like something... they will tell you the only way they know: by crying! And what happens when a baby spends a lot of time crying? They get very tired. Overtired. And is it easy to put down a baby who is overtired? Nope :(. It's a pretty evil cycle.
That is the main reason a baby will consistently not fall asleep until a certain time, but there are other culprits like overstimulation, lack of routine (check out my ebook if you'd like more direction with a bedtime routine), vaccines that week, teething, and then just plain old baby weirdness sometimes...
But before you put your baby down for bed or for a nap, make sure baby is tired!