1. A lack of a mechanism for using it for things like online purchases. 2. What about the 1% of the time when it is not instant?
On top of that you are not allowed, by law, to charge customers more for a card purchase so customers have no incentive to switch from cards and the existing mobile phone payment systems.
It does sometimes take two business days to clear, so it's not good if your service requires fast turnaround (e.g. pizza delivery). Of course, it's always good to have another option, and perhaps even for pizza delivery you could accept it on trust with customers you have a good relationship with.
(On the software side, unlike a credit card payment, this does require that you temporally decouple ordering and billing. Besides the processing delay, a lazy user might not send the payment immediately. Two weeks seems to be a typical length of time, and of course you don't ship anything until payment is received. Treat it a bit like a cash-by-mail payment.)
That's not the American way. Our services are both expensive and slow. You see, it's about freedom! We are the Most Free (lol, sorry) and therefore corporations are free to be bad in both ways simultaneously. :D
The key difference here is that you communicate with your bank, giving them a direct instruction to send money to the recipient at their bank. With cheque or card, the recipient communicates with the recipient's bank, to forward an authorization written by you, which is sooner or later verified against your bank. Which is convenient for you, but involves more steps.
I'm assuming "wire transfer" means something different to you than what it means in America. In the US, a wire transfer is when you call up your bank, give them a routing number and account number, tell them to wire however much money to it, agree to pay like $20 or something for the transfer, and then they tell you the transfer will happen tomorrow morning because it's 3:30 PM and the last batch gets handled at 3:00. They're not so much hated as pretty much entirely irrelevant to everyone. I've made one wire transfer in my life, and that was for buying a house.