Usually multiplayer betas are setup across the major platforms that the game is releasing on. One would expect that to apply to Titanfall 2 for Xbox One, PS4 and PC. But Respawn is skipping PC because they don't want the single-player leaked ahead of launch.
In a post over on the Titanfall 2 blog, Respawn co-founder Vince Zampella explained what their plans were for the multiplayer stress test for the upcoming FPS. Zampella admitted that Titanfall 2's beta would not be made available on PC due to the potential single-player leaks that would likely occur, stating...
Also, because the single player story has some wonderful secrets that we really want to preserve for launch, we also have to worry about leaks. Our wonderfully curious PC players have proven in the past that they will dig out anything they can, and there's a risk that they could spoil some of our single player.
That's actually a fairly legitimate reason not to hold a multiplayer beta test on PC. PC gamers usually data dump and then data mine limited run alphas, betas or whatever test is held on the platform to examine the code, the assets and other materials that go into making a game.
In fact, during the beta tests last year PC gamers dumped the Overwatch assets to have fun with them in other programs like the Source Filmmaker or Garry's Mod. In some rare cases gamers with modded consoles can also dump and leak code, which is how we found out that GTA V was being ported to the PlayStation 4 and also coming to PC, after hackers dumped, converted, and spread around the data files from the game after it launched on the Xbox 360 and PS3, two years before it ended up on PC.
In this case, Zampella wants to keep the single-player for Titanfall 2 intact, and they want to limit the amount of content that people are exposed to for the game before it releases this fall.
This is probably one of the more honest answers regarding why a beta isn't coming to PC. Let's just hope that they optimize the game well for PC when the time comes because according to Zampella they have not begun minimum and maximum spec testing yet.
In the post, the co-founder of Respawn Entertainment admits that another reason Titanfall 2 won't be beta tested on PC is because right now they haven't run the gamut of hardware configurations for the mech-shooter... yet. They plan on doing that later but, for now, they really wanted to stress test their servers and ensure everything works properly before the game launches, mostly to avoid over-crowding during launch or servers going down due to day-one activity.
While PC gamers may be left out of the loop when it comes to stress-testing Titanfall 2, I don't think too many gamers will mind so long as the game runs well on day one. Optimization is very important for the PC gaming crowd, and Respawn will likely want to avoid a scenario like No Man's Sky or Batman: Arkham Knight where thousands upon thousands of gamers have trouble running the game due to poor porting.
You can look for Titanfall 2 to go live this fall on October 28th for the Xbox One, PS4 and PC.