Most likely the price point they have set for the software is too low to justify the hire of a programmer to add this functionality. Serif has always set their software on the value end of the pricing spectrum. I can't see any other explanation for why 12 years later they still can't support Arabic language. I don't know in real life how big the company is, but many companies that do business on the internet look like big companies but in reality are just a small team of 2 to 10 people sometimes working out of someones garage. Now I'm sure Serif is bigger than that, they have global distribution even back when products were sold in physical CD, but that doesn't necessarily mean they are big enough to hire 1 or 2 extra programmers.
But if you look according to a quick internet search, Arabic is spoken by roughly 420 million people in the world. I am an Arab, I know for a fact that most of the market here is for cheaper value software and as soon as a solid alternative to Adobe products is introduced, a huge new market segment will be open to them immediately. These days you don't even need a regional company to help you market, you just flood google ads with marketing your product. That is a lot of software licenses waiting to be sold.