We know our mRNA vaccines are safe and they worked great against Delta. Also Omicron is not a vaccine-optimized Delta, it is completely different strain that has evolved from an early variant.
A vaccine targeting Delta probably wouldn't have fared better against Omicron. In fact, it could have been even worse.
While making a new variant-optimized vaccine is easy and takes about a week, and maybe they have already done it. But it still takes months to test it and to produce it at scale. One thing for sure, it wouldn't have been ready for the current wave.
I think one reason we don't have an omicron-specific vaccine underway is that we came out lucky with this variant. It is less deadly and vaccines are still effective against severe disease. If anything, the spread creates natural immunity. My guess is that the next big wave, which I hope will never happen, will not be Omicron but instead a new variant that escapes immunity offered by an Omicron pre-infection or vaccine.
I think people are now looking for a more generic anti-coronavirus vaccine, capable of protection against all variants of SARS-CoV-2, and possibly other coronaviruses like SARS. But it is not as simple as copy-pasting the genetic code of the spike protein like with current vaccines.