There's many posts which provide evidence for or against Present Mic being the traitor, so there's no need to repeat to what's been said. Instead, this thread is less concerned about 'who', but 'why'. One of the difficult parts about traitor suspects is pinning down a motive. Due MHA to having such a large cast, many character backstories or deeper complexities go unexplored.

Out of everyone though, Present Mic has the most development as he's a major character of the vigilantes side series. At the same time, he's conveniently still a mystery, as the readers never see his inner thoughts. For example, it went in-depth into Aizawa's grief about his best friend's death, but the readers ain't given anything about how Present Mic dealt with it. Also, it would be ironic for the main series, as the audience narrator turns out to be a traitor/villain.

MHA always make sure their major villains have ideological motives that relate back to the flaws in hero society; Tomura wants to prove Society's sense of safety is an illusion, Overhaul thinks society is better off without quirks and Stain thinks the hero profession is filled with corruption.

While there are villains like Toga who are motivated simply by their dark impulses - This motive would be considered a cop-out for the traitor. For a critically acclaimed series, fans expect the traitor's build up to reveal a complex motive. MHA would want to avoid the cliched "they were crazy all along and completely switch personas traitors".

Since Present Mic has an Intelligence grade of A, he could certainly spot flaws within society if he set his mind to it. So the question is, what specific flaw can be assigned to Present Mic? I think it's the participation of High School students in life-threatening hero work.

In Japan, the age of entering the army and police force is 18. Yet, Heros can routinely take 16/7-year-olds to fight dangerous criminals, whose often trying to murder them. It could be considered hypocritical of heros to claim they're the peak of justice, yet their safety measures for training school students are lower than the police/army. The most recent example is Tokoyami and Kaminari fighting in the front line against the Quirk liberation army, the equivalent of an actual war.

Present Mic was personally effected by this, when his friend, Oboro was crushed by rubble at 17 during their work studies. Even a decade later, when he's is a Teacher there, UA and wider society hasn't learnt much. Students who haven't turned 18 are still allowed to go on life-threatening work studies. In the Sport Festival, it's public entrainment to watch 15-year-olds injury each other on Live TV. Even cases like Izuku slowly breaking his fingers and limbs are acceptable. If Recovery Girl's Quirk reached her limits, a student could easily die one day. It wouldn't be hard to imagine some audience consider the Sport Festival battle to be problematic. Or in this case, motivate a villain to expose a flaw within hero society.

So how would this motive relate to the traitor's actions of leaking information and getting UA's students attacked? At first, it seems paradoxical with my theory for Present Mic's motive. However, not necessarily. Present Mic might consider the two set ups at the SJG and Summer camp "necessary sacrifices" to expose the issue to the media. Since then, there's been no further incidents of the League of Villains having information they shouldn't do. It's possible the traitor has ceased contact with the league.

In this theory's version of Present Mic, he would have achieved his goal of starting a public debate and doesn't want his students to "get hurt more than necessary". Especially since months after the two attacks, he discovered how the League of Villains turned Oboro into Warpgate and probably vowed to never work with the league again.

If UA, the highest ranking school in Japan can't guarantee safety, it could cause all of society to rethink how hero education works. It could prevent others having the same fate as Oboro, who died too young before he could even become a pro hero.

Bonus:

Since we're on the topic of traitors, I'd like to mention a suspect I've literally seen no one else consider. Is it borderline implausible? Yes. This is more for the fun of it. But regardless, when it comes to the traitors theories, everyone is a suspect and nobody can be overlooked. So Tokoyami is occasionally brought up in traitor debates, however, he's technically two suspects at the same time. His Quirk, Dark Shadow is its own separate entity afterall.

This can't be called a theory or even speculation, as there's no evidence behind it. It's just a random thought that popped into my head. But to elaborate regardless, I wondered if Quirks could develop their own consciousness separate from their user. Naturally, this made me look to Tokoyami, who' Quirk even controlled him during the attack on the summer camp. However, what if this was taken a step further - Dark Shadow controls Tokoyami's body without him knowing it, probably when he's a sleep. That way, the traitor would be right under the reader's noses, without even Tokoyami himself being aware of it. That could a reason for Mr Compress's abrupt decision to kidnap Tokoyami. After seeing Dark Shadow losing its temper due to the night time and jeopardising their mission, they'd want to talk to the rogue quirk immediately, to know what the heck it was doing. Which they could do without raising suspicion of Dark Shadow, by doing it under the cover of "kidnapping" Tokoyami.

It might even relate back to the Quirk Singularity, which the series is building up. Fans expects quirks to simply overpower their user's body, but perhaps it's step further than we expected, certain quirks becoming sentient.