Currently, Bitcoin isn't a currency for many people, it's an investment. People use fiat to buy BTCs, hold it for a good time, sell it and make profit next. That is what many people do. When more stores start accepting BTC as payment method this reality can change.
And there is another issue. The Blockchain doesn't support many users making transactions at same time, as you said. If BTC wants to be a global currency, with many users, it must change.
The most of bitcoin user treat bitcoin as investment like hold and sell, trading, or other investment in any business also developing new altcoin.
They keep use bitcoin besides their local currency, before use this money, the profit from any investment changed to local currency. Maybe fiat still have a special place in most bitcoin's user mind