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:

Rumors to the contrary, Rep. Brian P. Bilbray of California says Republicans have no plans to force Democrats into voting on an immigration enforcement measure written by one of their own.

Rep. Heath Shuler , D-N.C., has introduced legislation to strengthen border security and force employers to verify that their workers are legal (HR 4088). But the measure includes no provision to legalize millions of illegal immigrants, an idea that most Democrats say must be part of any immigration overhaul and that Republicans deride as “amnesty.”

Consequently, Shuler’s bill is more popular with the GOP than with his own party — 90 of its 136 cosponsors, including Bilbray, are Republicans.

Immigration lobbyists and interest groups suspect Republicans might try to force a vote on the bill by collecting signatures on a discharge petition.

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.