It's has a lot to do with the weakness of the evidence.

Edward II had a favourite knight called Piers Gaveston. They had known each other long before Edward II became king, and when Edward took the throne, Piers was made the Earl of Cornwall. It is the nature of the relationship between Edward and Piers that gave rise to questions about Edward's sexuality.

We can be pretty confident that Edward was not homosexual. He had an illegitimate son in 1307, which makes it a bit unlikely that he had no interest in women. The question then becomes one of whether he was in men as well as women, which in turn is a question of whether the relationship between Edward and Piers was a sexual one.

In the 1320s, an anonymous chronicle called the Vita Edwardi Secundi recorded that Edward "bound himself with [Piers] before all other mortals with a bond of indissoluble love", and some people (both then and now) have taken that as signifying a sexual relationship, and it is from this one chronicle that the idea of Edward and Piers as lovers originates. From there it spread, and some (but not all) later chronicles suggest Edward was a "sodomite". One even refers to the pair as "the king and his husband" as an insult.

But there are reasons to doubt the evidence. Firstly, Edward was not remembered as a good king. Much of his reign was blighted by famine, war, and incompetence that eventually resulted in his overthrow. Chroniclers of the 14th century were therefore somewhat hostile to Edward and if there was a way to make him look bad they'd do it. Portraying his relationship with Piers as a sexual one would have been an obvious way of making him look bad, as it would have added an extra layer of condemnation to the already egregious favouritism Edward showed toward Piers. Most of our other sources simply complain that Piers had unrestricted access to the king's favour, and that he paraded that access with extraordinary arrogance. In other words, the handful of sources that suggest something sexual between Edward and Piers have an axe to grind, so we maybe shouldn't take any allegations that would have been read as negative by a medieval audience at face value.

Secondly, the phrase "bond of indissoluble love" could mean a lot of things. It could be that they had simply sworn their friendship publicly, which was an accepted social practise that had been common among knights for at least 200 years at that point. It reads as homoromantic to us, but may not have to them. It could also be that they had become adoptive brothers, which was a less common but still accepted practise among the nobility. The big difficulty is that when the medieval nobility wrote to their loved ones, or about them, they tended to use the same language regardless of the precise nature of that relationship - a lover, a son, and a close friend would all be written about in a similar way. This is a massive pain in the arse for modern historians trying to work out medieval social circles and nature of the relationships in them, because in the 21st century we do change our language quite a bit. Circumstances of the current pandemic aside, it is normally regarded as a bit weird to openly and publicly tell a friend that you love them, miss them, and look forward to giving them a good long hug when you next see them. In the Middle Ages that was all completely normalised, which makes it very easy to see some of these medieval relationships as sexual or romantic when they probably weren't (or at least, wouldn't have been seen that way by the people in them). Friendship was a lot more serious among the medieval nobility than it commonly is today, and it was normal to use language like 'indissoluble love' to describe a long, close friendship.

So the reason most current historians don't think Edward II was homosexual is because the evidence has been recognised as weak. The idea can be traced to one main source that makes a rather vague claim about Edward and Piers publicly being close, and that's not enough information to make any strong assertions about Edward's sexuality. That's not to say that he wasn't sexually attracted to men, just that the evidence isn't strong enough to go "yes this man is gay and I'm confident enough to put that in my academic work".