For me I'm TTC in my late 30s (2nd baby) as I didn't meet Mr Right till late.
I imagine the rise in house prices might have something to do with it. If people are focussed on earning enough to buy property, it takes so long these days to get a foot on the ladder and this may be having a knock on effect. In my parent's day, if you had a degree you got a good job, then could most likely afford a house. These days if it's important to you to own your own home before starting a family then you'll have to wait so much longer (obviously only talking about those for whom home ownership is a possibility!)
Also, there have been massive changes to the way we live, within living memory.
Way back when, people used to get a job for life, and - it seems from here anyway - that life was more structured. You got a job, then married, then started a family.
These days, people change jobs more often and have less job security, we also have more relationships - with marriage often not happening until after we've had DCs. We're more used to being consumers, and we're less religious. Perhaps that means that we're more used to the idea that we can have the power to exercise choice in our lives, which leads us to be picker about our partners, rather than marrying younger and then living with it to avoid the stigma of divorce (which of course is less of a stigma these days).
Women especially have more options open to us, and many of us don't feel we're expected to have DCs anymore, it may take a while to decide that actually you do want to. Especially when you realise it's now or never.