Its difficult because it requires parked money (nostro accounts) in foreign currencies. This imposes a risk and act as "dead capital". At some point the cost to maintain the corridor is higher than the profit so the corridor is closed. Transaction then are routed trough other corridors which means multiple currencies swaps. More loses and more parties who want their cut. + it can takes days and the system are one-way so you have to ask the recipient if he got it to know.

Public ledgers can make a difference See https://ripple.com/ripplenet/on-demand-liquidity/