The problem with those are that they're more complicated (even when perfected) than current treatment. Immune modulation would require either suppressing the entire system (or very large chunks of it) which is done with some other diseases but generally has super high costs, as it leaves you quite vulnerable. Or you're targetting only the handful of cells responsible (my DM I knowledge is rusty but I'm guessing b-cells?) in which case you're on your way to solving a bunch of different diseases.
Stem cells has a similar issue, in that once you made new beta cells your immune system will just react to those.
Current treatment is quite alright. There's ground to be gained in earlier prediction and treatment, and a ton (this should excite people here on HN) in bettering the treatment method—create a proper feedback loop with insulin/glucose readers that automatically push out insulin when needed instead of a huge shot of it when you're guessing you'll need it.