From the administration's response: "As a part of immigration reform, the Administration strongly supports legislation to attract and retain foreign students who graduate with advanced STEM degrees, to establish a start-up visa for foreign-born entrepreneurs to start businesses and create jobs, and to reform the employment-based immigration system to better meet the needs of the U.S. economy. However, the Administrat…
Because they want to force Democrats to be on record voting against letting entrepreneurs into the country. The potential for attack ads during the next election cycle matters more in Washington DC than actually getting things done.
Also, while this is framed as an immigration bill, the Democrats won't take too much bad PR in important (Hispanic) immigrant communities because the vast majority of those aren't the tech job type.
Holding out for a comprehensive plan is probably a good move for Obama because he can include this as an incentive to vote for it. If they don't get some sort of comprehensive plan, it's still good because the Democrats could basically just introduce this without the zero-sum part and then the Republicans will be the ones against both skilled and unskilled immigrants.