Lots of reasons, most of which are misunderstandings, in my opinion, when it comes to Christianity, anyway.
People don't learn about religion in the same way they used to, so they make up their mind based on assumptions without taking the time to find out if their assumptions are true (e.g. re homophobia, role of women).
Society values things which are at odds with the message of most religions, e.g. money, status.
People focus on what they'd have to 'give up' rather than what they'd gain.
'New age spirituality' makes people believe they don't need God.
People think science can explain everything.
I think Christianity is just as relevant today as it has always been if you actually read the Bible as a whole, rather than take random verses out of context, and are willing to have a two-way conversation with God through prayer.