Why Some Democrats Refuse to Investigate for Impeachment

Because some of the investigations would inevitably lead right back to some of their own members having been complicit in criminal actions:

It seems as if the "four" congressional leaders Harman refers to as knowing about the tapes were the chairs and ranking members of the intelligence committees: Sen. Pat Roberts (R-KS), Sen. Jay Rockefeller (D-WV), Rep. Porter Goss (R-FL), and Rep. Jane Harman (D-CA). Rep. Pete Hoekstra (R-MI) took Goss' spot as chairman of the House intelligence committee that year when Goss became CIA director. Hoekstra told the AP that he didn't know a thing about either the tapes or their destruction. I'm calling Harman to ask her for her letter to the CIA about the tapes, and will bring it to you if and when I have it.

But the bottom line here is that at least some Congressional leaders knew something about the tapes and something about their destruction, and didn't say anything about either. Harman's silence is especially stunning: she co-chaired a joint Congressional inquiry into the 9/11 attacks in 2002 that didn't receive that very pertinent information. Why did she remain quiet about potentially criminal behavior? Marty Lederman has some thoughts here:

Jay Rockefeller is constantly learning of legally dubious (at best) CIA intelligence activities, and then saying nothing about them publicly until they are leaked to the press, at which point he expresses outrage and incredulity -- but reveals nothing. Really, isn't it about time the Democrats select an effective Chair of the Senate Intelligence Committee, one who will treat this scandal with the seriousness it deserves, and who will shed much-needed light on the CIA program of torture, cruel treatment and obstruction of evidence? ...

Jane Harman also knew of the intention to destroy the tapes, and she at least "urged" the CIA in writing not to do it. (Where were her colleagues?) But when she found out the CIA had destroyed the tapes, where was Harman's press conference? Where were the congressional hearings?

You are either with bush OR you are against him. If you enable the bush administration to cover up their crimes, you become part of the criminal conspiracy.