Grammar with explanation

如果佢地已經完成developing, 你就會寫 That's the reason why we have developed the new version of it.
如果仲係develop緊, 就會係 That's the reason why we are developing the new version of it.
如果用 That's the reason why we develop the new version of it, 可以解佢地時常develop新既version.

你可以用why, 或者唔用都可以. 用既話就會順口D. 平常同人講野既時候都會用. 如果你寫既時候唔用, 聽落去就會好似好sophisticated. 其實無咩所謂~~

係需要用"the", 或者可以用"a". 如果你想講關於一個specific既version, 敢你就用"the". 如果你係講緊其中一個version, 就用"a". 唔用既話, 聽落去就唔係幾順口.

希望幫到你啦~ :)

That's the reason (why) we develop the new version of it.

I believe that 'why' is an option . Using simple present sense for 'develop' as it is a 'fact'. You should not use are developing as your development has been completed and you
have stopped doing it.

You need an article (a,an,the) before the word 'new'. It is more appropreiate to use 'the' instead of 'a' as you mention that there is an existing product that related to your new develop product.