Because according to the Fox News report on this, about 2 months ago, The DEA was finding its investigations into massive Israeli ecstacy and other drug rings being continually foiled. They found out that these drug gang/spy rings had all communications numbers and were listening into all DEA and intel agency communications. This was assisted by an Israeli communications agency which somehow has gotten the contracts to log all phone, cell phone, pager, etc communication records in U.S. (sorry dont have name but it was in FOX's report)
That was how they tied these groups activities to the supposed Arab groups in the U.S. involved in 9/11 attacks. They found a pattern of the Israeli groups seeming to mirror or monitor the Arabs groups that their 9/11 investigations exposed.
This also helps to explain why there was a group of Israelis celebrating on a rooftop in New Jersey, watching the Towers burn. They were reported and turned in on neighbors suspicions. They were held for awhile, I'm not sure but I think they were all deported.
The good news here is that, if my surmise is accurate, the DEA ran across non-DEA related info and passed it to the appropriate agencies. This might not have happened pre-9.11.