Because most people who have a choice won't choose mode of transport that is both unreliable and expensive. Which most public transport outside of London is. For example we visited my parents yesterday. Fairly easy drive of just over an hour each way. Had we gone by train, it would have cost £70 for the (two) tickets, taken 2 hours 20 each way and to get to the station taken at least another forty minutes and cost another £5 each (so £10) for a day pass. So that's an additional 4 hours travelling time altogether and an additional £50 cost. No one in their right mind would choose to do this.
Why are we fucking around with initiatives that make driving difficult instead of initiatives that make public transport cheaper and better?