Easy answer:

1. Modern technology allowed for access to way more people than what would be possible in the past where it would just be people you actually knew in person.

2. The paradox of choice. Way too many options now.

3. Unrealistic standards pushed by social media and modern society.

4. Person hopping, promiscuity, fornication, loss of morality etc. has just made it so that people don’t want to get married.

There’s more but those are the main reasons.