Because it needs time to grow. The general public are sadly generally ignorant and all technology takes time to get accepted by the masses. The internet or personal computers didn't just blow up over night. They were created by geeks and used by geeks but the mainstream realizes a great invention eventually. I think once the remittance/gambling/sex markets realize how much money they can save and the users realize how they don't have to give all their personal details to these gambling or sex sites the benefits double. We just need time.
How much time should pass until we begin noticing changes? Personal computers has been around since the '70s, but they didn't make it into the masses because of the price primarily (and software somewhat lagging behind). There are no technological obstacles that could prevent bitcoin widespread adoption, so your examples are irrelevant. Visa and Mastercard transfer money even faster than blockchain, and how many people need online gambling or sex (beyond family)?
You wrote
"
Visa and Mastercard transfer money even faster than blockchain
"
WRONGVISA and Mastercard give you a message they are taking care of the transaction.
The transaction is not finalized and irreversible until many weeks later.
If you receive a payment using the VISA or MC networks, you can not spend the money before the money land in your bank account.
And this take time, a lot more than 6 confirmations.
After one hour you can spend your bitcoin.
Technically one could spend them after one confirmation, if the receiver trust you and the network.