Broadly speaking, because the credit card industry is a massive, bloated, inefficient mess, and the effort and liability required for a small business to participate in that mess directly can literally be business-destroying. And that's assuming the Powers That Be graciously allow you to play their little game at all, which may take several weeks of approval process to decide, and will almost certainly come with silly levels of fees on each transaction and terms so one-sided they could right a sinking ship even if you do jump through all of their hoops.

Having an intermediary that can provide a simple API that just works instead of having to integrate with some absurdly complicated and poorly documented API offered directly by the big players can save weeks of hassle. Similarly, having an intermediary that can take care of compliance with the dreaded PCI DSS (the industry security standards) is also a huge win. Finally, because the intermediaries tend to be bigger fish in the pond, you will often get a significantly better deal out of them in terms of how quickly you get your money, and how much of your money you actually get to keep.

All of this is particularly true for start-ups with no trading history, which of course will include just about any new on-line business. Once you've got a proven track record you can give solid answers to the financial questions and actually make the big boys compete for your business as they should, at least to some extent, but if you're starting from zero then the first thing a lot of these companies see is a huge potential fraud risk, particularly if you're in any kind of "dubious" industry or, again like many on-line SaaS businesses, you want to use a continuous payment authority to take recurring payments from a customer's card and not just a one-off transaction.

Unfortunately, there has to be some sort of big player on the other side of any intermediary (or, possibly, several big players used for different kinds of transaction) and so it's not unusual for these intermediaries either to be limited in geographic scope or to provide a partial service but still require you to sort out your own connections with a payment gateway and/or merchant account based in your region (though the intermediaries will probably be able to help with that process, which is still better than nothing). Thus while the US appears to have an effective and fast-growing young industry in payment processing services, those of us in other regions can only look on with jealousy and bow down before the mighty PayPal/Google/other vast multinational with a self-contained payment service.

As someone who has credit cards from 3 different countries, I can tell you that's very wrong. I'd like to think my CC works anywhere, but oftentimes when they ask for a ZIP and I don't have one, that's a deal breaker. Sometimes when the person sees the card is from another country (name of country is in the bank name) they refuse to use it, even when it matches my accent and driver's license.

Also it would have been nice if VISA had a global "credit rating" instead of a separate one for each country. Unfortunately, I now have zero credit in my new country, which is painful when I have amazing credit in my previous country. (all VISA cards)

Pro-tip: if you deal with a bank that has branches in both countries, and you're sufficiently good of a customer that they can justify an hour of work for you, you can get around not having a credit history in your newest country of residence. CitiBank, for example, has an employee in Tokyo whose only job is being attached to an Experian terminal to make credit decisions for American expatriates. (It is highly likely that branch level employees at most CitiBanks are unaware of his existence, so the high-percentage plays are either speaking to their wealth management group, speaking to their "executive relocation" group, or sending the bank a letter.)

Hosted PayPal is country-agnostic, but it looks amateur hour.