The ball goes through two phases at impact. First there is a slip phase and then there is a roll phase. The sooner the roll phase begins in relation to the slip phase, the more spin you create.
The optimum slip/roll sequence for spin happens at "around" 58 degrees dynamic loft....all other things being equal. If you don't have much forward shaft lean, then you are flirting with that threshold where the spin drops off and the launch increases disproportionately. Obviously, if you're at the "top of the curve" like that, you may experience a wider range of trajectory (low spinners mixed with high knuckle balls), and thus distance control.
Then you ask me: "well gee wiz, I'll just lean the handle more forward!" Not so fast sparky, if you lean the handle forward, this is usually in tandem with steeper AOA, so you are making a more "oblique" strike on an already "oblique" loft. Thus, your hit location is more likely to vary up and down the face....which affects launch, spin and speed.
Now, the other reason would be that, from a playing standpoint, golfers are more likely to hit partial shots with the 60 degree than any other loft in the bag, and of course, based on that you are likely to see a larger dispersion in distance control.
And finally, the cg/sweetspot on a typical forged 60 degree is very high on the face and heelward. The type of lie you have has a huge influence on your ability to get the impact up into that sweetspot......which has a huge effect on launch, spin and speed. Off a tight lie, you are going to be hitting it "thin" to some extent.