They are cheaper to run if you can charge at home (or work etc), which not everyone can. Electricity from a garage can cost nearly as much as petrol per mile.
However, unless you have been given the car for free, you have to factor in the purchase or lease price. And like for like, electric is much higher than petrol. As I said above, my £8k petrol car was £19k in the electric version, for a car only a few months newer.
Now I don't know about you, but if I'm comparing two very similar things that do the same job but one costs £11k more than the other, I'm going to have a really good think about whether that extra £11k is worth it, and how long it will take for the cheaper running costs to add up to £11k. For me, it was about a decade so I decided it wasn't worth it.
Now, I'm not completely against electric cars, although I do have some reservations about the viability of the infrastructure, as evidenced by all the queues at service stations over Christmas because lots of people were travelling longer distances in their electric cars and also the fact that as a nation, we don't currently have the generating capacity for a massive increase in demand for electricity that would be needed for 'everyone' to convert to electricity. So my current hope is that, in 10-15 years time, when the time comes that my little petrol car needs replacing, that the infrastructure will have developed to match demand and there'll be a decent scrappage scheme in place to make the cost look sensible because, if not, I might be looking to get one of the last newish petrol cars in 2029 instead to buy some more time before electric is forced on me.