I suppose they may be waiting for a more politically-opportune time. Once Senator McCain clarifies his stance — specifically, would he oppose any ‘comprehensive’ bill that may pass the House as a result of this debate — they will better understand the political calculus:
If a majority of the House (a total of 218 Members) signs a discharge petition, the leadership is required to schedule a prompt vote on the measure. The vote need not be simply up-or-down, the House would likely consider amendments and alternatives. So if/when Republicans decide it’s in their political interest to have an immigration debate, they could quickly get about 200 signatures. They would then pressure moderate Democrats, red-state Democrats, and swing seat Democrats to sign as well. The ‘right’ time to press for a vote would probably be shortly before the election — when there’s the greatest potential to either force a wedge between latinos and the Democratic party, or to show targeted Democrats as ‘soft’ on immigration. If the Democratic leadership is wise, they won’t wait for a discharge petition to force the debate on them, but will hold it when it suits them.
