I should imagine that the primary reasons will be access to land and/or expense of guide and/or rifle. How do you even get in the situation to shoot your first deer? If you have loadsamoney, you can just pay for stalks. You'll still need to shoot straight, but then you can pay for training beforehand. Otherwise, it's a tricky (and extended) business. How can you do it as cheaply as possible?
Things are slightly easier if you can shoot already. This is because then you "just" need to find someone who will take you out on his permission with his rifle to get started. Cadets, family, pottering around with an air rifle, etc. can teach you to shoot for free or cheap. This was my way in. I got paid to learn how to shoot in the army and then got introduced to stalking by a friend who is a guide and who owed me a favour: Total cost = 0.00.
Of course, you could technically shoot your first deer for "only" the cost of your FAC, binos, knife, rifle, scope and ammo (min. £500?), *if* you have access to land, but getting a permission without an open ticket? Good luck. Getting your open ticket without DSC1 (or experience, in which case you will have already shot a deer or two!)? Likewise. Maybe you get permission with a closed ticket? Local force going to ok the land without your DSC1? Hmmmm. So, getting your DSC1 without money? Ahem. Getting your ticket at all without DSC1, experience, or a booked stalk? LOL.
So, min. cost without some luck will be your DSC1, your FAC, and £500+ of kit. This, I should imagine, is why most people who want to don't get to shoot their first deer.
But, shooting your first deer? Priceless. You remember that forever.