Considering that the revenue per employee number is at an all time high, and the amount of money the company spends on the employee's working environment[1] is at record lows pretty much explains why this is going to be effective now when it wasn't before.
There is also the issue of the "big" tech companies piling up cash rather than compensating their employees. So giving a voice to that choice could be interesting as well.
[1] Most people I've had the discussion with will argue that "free food" and "bus service" is way more than employees in the 90's got. However on a per-employee basis, the typical professional employee (engineer) had 150 square feet of office space that was unshared, in an open plan office that is typically less than 18 square feet and can be a small as 9 square feet. 100 square feet in a class A office building is $1000/month for $10/sq ft. Food and bus service per employee totals to much less than that. The key is that with a union, the employee has a voice in whether they get compensated in office space or in free snacks.