I don't understand your question.

Different languages have different writing systems. Almost all of the the decisions about that predate computers by decades or much longer. The only answer can be "because of tradition".

Of course the first computers most easily encoded the English alphabet because they were made by English speakers. But with Unicode and other technologies, almost any language can be used on a computer now. There is no real problem, except sometimes software compatibility. And so now the answer is "because instead of changing the languages or their writing system, the computers are now more advanced and change to encode them better."

I agree about Vietnamese. (Though for some people it seems adding accent marks in Spanish, etc., seems like a lot of work.)

But I don't know if I agree with this:

Compared to Arabic, Hindi, Hangul or Chinese, Vietnamese is simple.

Chinese is complicated because there are several possible input methods (mostly either the traditional stroke-based system, or the more recent phonetic-based system), and Japanese similarly because there are multiple possible characters for the same pronunciation so it can be a little complicated.

But for Arabic, Hindi and Korean Hangul, they're just different symbols for pronunciation and aside from the computer knowing how to represent those symbols (and in each case changing the shape in certain ways to connect the letters together), typing them is quite easy once you remap your keyboard. It's basically the same as typing in Cyrillic or Greek alphabets, just different symbols than Latin. Learning the languages can of course be difficult, and the scripts too, but typing in particular is a pretty easy skill you can pick up in a couple weeks during your first language class, if you want, once you're familiar with the script. In fact, if you map your keyboard phonetically (but maybe not in the standard way for the language) you can actually "sound it out" on the keyboard and have the text appear onscreen. Or use an onscreen keyboard preview to memorize the arrangement that way.

Anyway, not an especially important point to make here, but I just thought I'd follow up because, once you remap your keyboard (different operating systems have various options, but they do all have options, usually by default if you know where to look), actually typing is really not hard at all.

Of course for any language using special characters sometimes it is hard to type when you don't have a your computer (or phone, etc.) set up properly, so accent marks get skipped, and sometimes languages are transliterated to Latin from other scripts. Typical of 9-button phones used for texting, although smartphones are making that less important now.

That's true, actually especially from the perspective of the native speakers who (in theory) might not know how to type in English, because it's two steps. It's not much harder for a learner, but requires speakers also know the Latin alphabet (not much of a practical problem these days but still).