One reason is because they've found that simply boosting with the original mRNA gives a massive spike in protection even against Delta, apparently on par with variant specific versions of the vaccine.

Another is that recent studies are showing that natural infection provides much more comprehensive, long-lasting immunity against all variants than the vaccination does, while vaccination with the current vaccines offers much more immunity than individuals who have never been vaccinated or infected. So it would seem like the best course of action moving forward may be to vaccinate everyone possible initially to prime their immune systems and reduce the severity of the disease, and then allow Covid to run its course. Eventually vaccinated individuals will have break-through infections, gain the more durable natural immunity, and Covid will become a less serious, endemic virus more like the flu or cold.

In short, we don't yet have a need for a Delta variant vaccine. That may not be true eventually, especially for future variants.