First this is not at all true that nowhere had a word for tea prior to this. Even if it were true, native words get replaced all the time. All the time. So if 100% of languages in the world used the same word for a single concept, that does not mean that that was always the case.

But aside from that, there are not in fact only two words for tea. Many other languages have words for tea that are not related.

Actually we need to step back a bit, because tea and cha are not different words. They are the same word, written 茶, formerly pronounced /dra/. The /r/ after /d/ in languages like Mandarin caused the /d/ to become the <ch> sound, while in languages like Hokkien the /r/ fell out completely. This is a bit of a simplification but since it's primarily to correct an error in the premise, I think that's probably sufficient. If someone wants a more detailed look at the historical development of the word, that can happen in a follow up.

So, one word for tea that got borrowed differently in different languages and at different times, kinda like guarantee and warranty. Some languages use a cha-like form, some use a tea-like form. Some languages like Arabic use both depending on where you are, and some like English use both depending on what type of tea we're talking about. Vietnamese also has two forms with two origins, with trà in some words and chè in others.

But these two (or this one) words are by no means the only way people say 'tea':

• In Burmese, there is လက်ဖက် /lapʰeʔ/ as the native word for tea. ကြမ်း /ʤá̃/ also came in via Chinese, but is used more in compounds than as a generic word. Mon, spoken further south, uses လက်ဖက်1 /ləphɛk/, related to the Burmese word.

• In Shan, a Tai language spoken in Upper Burma, there is ၼဵင်ႈ /neŋ³/. Sgaw Karen has နီမုၢ် which I believe should be pronoucned /nim˥˩/ and is probably related to the Shan word, though Karen is not related to Tai.

• In Kachin, a.k.a. Jingpaw, a.k.a. Singpho, a Tibeto-Burman language in the Northern Naga branch (a.k.a. Sal languages), there is /pʰəlap/. In fact in many of the Sal languages the word is something like /pʰəlap/ or /pʰəjap/.

So there are multiple language families that each have their own words. In fairness, the Kachin and Burmese words share what is almost certainly a cognate morpheme lap, but still none of these are related to cha.

What you should see that all of these languages have in common is that they are all from areas where tea has been grown for a long time. There is of course tea grown today in a wide range of climates, but the greatest concentration is in the area where China meets the rest of Southeast Asia. This includes Assam, of course, and it is in Assam that the British Empire established substantial tea plantations in the early 19th century that remain today. But of course Assamese just uses চা /sa/, which is related to cha.

The other commenter mentioned that China forbid growing of tea outside their borders. This is nonsensical. Tea has long been grown outside China, and the reason we have these two words doing so much of the work has nothing to do with state control and everything to do with the fact that Southern Chinese have long had complex mercantile networks throughout Southeast Asia and through these networks came into contact with early European, South Asian and Middle Eastern traders. It wasn't that the Chinese had any true monopoly in terms of production beyond the fact that, with more land, they simply had more tea to grow, and the tea being grown in mainland Southeast Asia was generally inaccessible to the British. It wasn't really until the 1920s that they covered any real ground in the area through a series of topographical surveys, so they certainly weren't establishing significant shipping infrastructure that far inland. The cultivation of tea in Assam was not to break a Chinese monopoly so much as to simply have a greater supply of cheaper product. Tea quickly took over much of the worthwhile land in Upper Assam, as is still the case.

In other words, there just wasn't a massive tea trade from Myanmar to England, nor were the early introductions of tea to the English happening in Old Mandalay. Otherwise we'd probably call it lapay. Indeed the earliest records of Tea in Europe called it cha or something approximating cha, as well as something like tea in other early cases, and a "standard" English name for it was not immediately settled upon at all. The earliest accounts in English were by people who'd set up camp in Cantonese-speaking areas and so the word they used was of the cha variety. It was only later that tea ("tee") came about in English, and that gradually replaced cha mostly because when you start hearing about something the way you hear about it takes over even if an older word existed that you didn't really know about.

However, regardless of the word used, it was through trade with Chinese states that the majority of places outside Southeast Asia (to include England) got their tea, and thus we are using 茶 to refer to it. Meanwhile in Southeast Asia where it was already quite common, there are a dozen other words in use.

The words aren't powerful. The paths that brought tea into these langauges are, and it just so happens the major trade powers in the world at the time were working with other major trade powers, like China, and skipping a lot of the smaller players who had very different words.

As a quick aside, there's a map and associated article that's been going around, called like Tea if by sea, cha if by land. It's not great. Please don't post it in reply to this thread. It's factually wrong on a number of points and minimalistic to a fault on others. It's the Guns Germs & Steel of tea word maps.

Also if you can't see any of the letters I wrote in those langauges, go install Noto Sans Myanmar from Google Fonts. Most fonts don't play well with Karen, let alone Shan.

(edited for formatting)