What's stopping them, if not our security services?
Is there? How do you know this?
The embassy bombings, the attack on the USS Cole, the WTC bomb attack, and the Oklahoma City bombing were all notable terror attacks on the US in the 1990s.
What terrorist organization wanted to attack us in the eighties?
What organization wanted to attack us in the nineties, other than the failed WTC bombing?
If that had succeeded, history might be very different. Success breeds success and the second WTC attacks on 9/11 inspired other terrorists.
Hezbollah (usually as "Islamic Jihad"), the Italian Red Brigades, the Red Army Faction, the Abu Nidal Organization, and the Japanese Red Army are among the foreign groups that not merely wanted to, but actually succeeded in carrying out attacks targeting Americans in the 1980s
The Armed Resistance Unit, The Order, and the Jewish Defense League are among the domestic groups doing the same.
> What organization wanted to attack us in the nineties, other than the failed WTC bombing?
This little group called al-Qaeda that you may have heard of carried out several attacks targeting the US in the 1990s, including the WTC bombing which killed 6 and injured 1,042.
Saudi Hezbollah also conducted a major attack targeting Americans, and there were several by Islamists for which no organization took credit or has been ascribed blame. This, of course, doesn't cover everyone who wanted to attack Americans, just those who actually carried out attacks that actually did.
There were also notable domestic terrorist attacks in the 1990s.