That doesn't make any sense. You want to prep a fix for distribution ASAP, and release it so people can be protected from it as soon as possible.

Also many attacks are not really feasible to detect if it's happened - that's like trying to know if the mailman read your postcard.

You can develop executable scanning techniques, but people are a lot more concerned about preventing it in the first place than catching it in the act. Why would you leave people vulnerable so they can watch data get stolen - instead of just fixing the issue?

Stop inserting random punctuation for emphasis! Periods go at the end of a full sentence.

They probably concluded it wasn't possible in this case. From their FAQ:

[Q] Is there a way to detect Downfall attacks?

[A] It is not easy. Downfall execution looks mostly like benign applications. Theoretically, one could develop a detection system that uses hardware performance counters to detect abnormal behaviors like exessive cache misses. However, off-the-shelf Antivirus software cannot detect this attack.

It was reported to Intel a full year ago, apparently.

The detection strategy is: are you using the affected hardware?

Not all systems are equally exposed, and while there is no universal low level fix many systems can be redesigned at a higher level to reduce exposure.