Funding and an integrated travel plan. It's where metro mayors and the like come in handy.
Also a less London-focused national budget would be a great boost to the regions.
My answer's going to be different depending on if you're asking for barriers to use or policy issues.
As far as use goes, transit needs to be frequent enough to be useful and go to where people need it to. It sounds simple put like that but you hear a lot about once daily bus service to some of these towns and then everyone wonders why we're so car dependent.
Access is also important and often overlooked in these discussions. Because of course disabled people who have no ability to drive will use transit. Para-transit style services have such heavy limitations that in many areas it can't be used for work and sometimes not even appointments. It keeps people who otherwise might be working out of work which benefits no one.
Policy wise, funding. It's a chicken and egg situation. No one wants to fund poorly used service, and no one uses the poorly funded service because it isn't fit for purpose.
Frequency and density.
The issue is that frequency and density is low so people become drivers and see transport as more expensive because they already pay the expense of car.
After that it would come via cohesion of the network: fares, navigability, timetabling etc. The fact first group has tap and go and then diamond has its own tap and go that doesn’t sync makes using transport a huge pain in the backside, similarly timetables are on different apps and places making it hard to navigate.
After that, it would come to the cost & hopper fares. Hopper fares allow for rationalisation of the network, lower fares could encourage usage. I like Scotland's idea of giving under 21s a free pass also, market subsidises it similar to the OAP pass but at times younger people are more likely to use.
I'm traveling to Manchester this weekend with the family.
I already have a car, so fuel would be about £70 or the train for 4 of us would be £400. With a travel card and it's silly restrictions meaning we can't leave early would drop it to £200 (plus the travel card itself because it never works out cheaper for us so I never get one), which is still more than twice as much as it would be to drive. I would prefer to sit around a table with the family for the ride, but not £400 much.
My car is off the road at the moment so I've been looking into public transport alternatives. For reference, driving to work is 10 miles, takes 25 minutes and costs about £2 in fuel each way. I pay around £1.20 for parking at work.
To go by bus, it would be 3 buses, take 2 hours and cost £4 each way (there are a few different options but they all work out the same). The train could replace one of these bus journeys and reduce the time to just over 1 hour but would cost £10 for a day return.
Both of these are only available once an hour so depending on what time I need to be at work, I might need to leave up to an hour earlier for my commute and it would still cost me twice as much as driving.
I'd be willing to pay a little more because I could work/read on public transport, plus getting rid of the car would save maintenance, tax and insurance costs, but double for the bus (or triple for the train) is excessive.
However, the major issue for me is the time - I already spend the best part of 1 hour commuting and this going up to 5 hours is not workable.
Alternatively, I could move close to my workplace, however house prices in the city are at least double what they are here and rent would be 40% of my salary after tax.
The issue stems from everything public transport being in a star network, I have to get a bus to the nearest town, bus/train to the city of my workplace and then bus to the workplace. Even though I live in the next town over on the side closest to my work. My workplace is also on this side of the city.
Reliability, price, and the public.
Thankfully, two of those are usually ok in my neck of the woods.