Which English King was the first to speak English as his primary language, and not French? Why the transition?

Edward III, who definitely spoke French as his primary language, was the king to issue the Statute of Pleading in 1362. This was a law that required English to be spoken in the courts because otherwise the vast majority of the population wouldn't understand what was going on. Previously thy had spoken Law French. (Because the English are generally loathe to give up on anything Law French remained until the 16th century for all sorts of purposes but common pleas had to be in English from this point forward.) So, although he wasn't the first to have English as a primary language I think he's an important place to start.

Richard II spoke English very well and presided over the flowering of the English language in the 14th century. Although he was not one of the main patrons of the poetry of his time he would certainly been aware of it. It is unlikely that English was his primary language (so not the answer to your question!). But he was definitely very adept in it, which I think is an important thing to note.

One of the big patrons of that 14th century flowering of the English language was John of Gaunt, Edward's son. He was the uncle of Richard II and father to his cousin, Henry IV (who would usurp Richard). John is famously Chaucer's major patron and they were even in-laws toward the end of Gaunt's life.

Another person to benefit from Gaunt's patronage was John Wycliffe, who founded England's first "heresy". Wycliffe attacked the clergy much as Martin Luther would later but more important to this is he was the first person to translate the Bible into vernacular English. (Wycliffe's heresy, lollardy, was persecuted in England but Wycliffe survived due to Gaunt's protection.) I think this is important because when Wycliffe wanted to translate the Bible into a vernacular language he did so in English. Which meant there would have been a good number of people who could read in English. People who would prefer to read in English.

So when Henry IV took the throne, being the son of this great patron of the English language, he took the oath in English. Although he would have been fluent in French, he certainly used English in his day to day and I think he's a safe place to say really began to emphasize English from the throne. And, as stated above, his being raised to focus on English was likely a conscious choice by his father.

As to why? Well, I don't think you can state one answer. The Hundred Years War played a part. Even as the kings of England were trying to become the Kings of France there was a lot of animosity toward France that grew up from that. And the nobility were increasingly more associated as English.

But that is a superficial answer. It goes back much further. I think you can take it back to Henry I (son of William the Conqueror), who married a woman descended from the Anglo-Saxon kings and made conscious efforts to incorporate English words and ceremonies into his court. Thus his son was called William the Atheling (meaning prince). His nobles sneered at him for this and it certainly didn't catch on with them. But I think it's really the first sign of an attempt of genuine integration. The first ember of a joining of the people not just a group of conquerors and their conquered. And everything has to start somewhere right?

I think you can see it with Henry II too. Who wouldn't have spoken English but did try to incorporate, regulate, and enhance English laws and customs. The courts were all in French but something distinctly English was growing up under Henry. Slowly. Again, this is not anything close to transitioning to English but it's a journey.

Most importantly, under John when they lost most of their French possessions nobles, who previously may have had land on the continent and in England, mostly had to make a choice. They could be English or they could be Norman. Suddenly their identification with England was much more exclusive.

Under Henry III there was a lot of growing resentment and hatred toward foreigners especially from Poitou. By this point, you have a sense of English barons being totally separate and distinct from the Normans or Angevins. And proud of it.

Slowly through out this time you have a formerly Norman nobility becoming increasingly anglicized. They may still be speaking French but they will need to know English to communicate (in the past anyone worth talking to would speak or learn French).

At the same time, there was a growth in trade in London, and the growth of the language was heavily influenced by the London dialect (although not exclusively so). So you have a small but growing class of wealthy or middle class people who grew up speaking English. These aren't nobles but they matter. And they would have learned French as they became more important (or tried to) but they would have spoken French like someone from London does. Which... is not really French. Chaucer makes fun of such people in the Prioress's tale. Chaucer comes from this class of people.

So, yeah, by the time they were deep into the Hundred Years War there was a lot of hostility toward the French. But it's not like the nobles were like "eff you I'm not even speaking your language anymore. Take that!" It was a gradual process, almost tortuously so, going all the way back to the generation after the conquest. It reflected the increasing anglicizing of the nobility. And, of course, the increased Normanizing of the commons as Middle English was a very different animal from old English. It definitely went both ways. And a king, in the kind of personal rule reflected in a "feudal" culture, can't be so far from his barons that he doesn't speak their language.