This therapy maybe the simplest of all the therapies, if the body has low levels of some enzymes let's produce some and inject them into the body. Since the cells are adept at picking up enzymes from outside the body and absorbing them it seems like it should work. Gene therapy is really just a complicated form of ERT - we are attempting to add an enzyme (Hex A) into the body. Cord Blood transplants and stem cells are also therapies attempting to add enzymes. Of course adding enzymes is of no use if they don't remove the GM2.
This therapy has been very effective with some lysosomal storage diseases (Gaucher disease, Fabry disease, or MP1) - but not with the disease that involve enzymes needed in the brain. The blood brain barrier is a factor, the foreign object response of the white blood cells is a factor, and the quality of enzymes needed is a factor. Additionally, some ERT treatments only impact symptoms temporarily as the injected enzymes will eventually die off - that is a problem for applications as invasive as direct brain injections.
In therapies like gene therapy the number of cells injected is lower because the cells duplicate once they arrive on site. In cord blood transplant therapies the blood continually regenerates Hex A. With a therapy like ERT- we need to put in as many as we need, and the process may need to be repeated several times it might be able to slow the progression but it cannot be a permanent solution (at least not yet).