CRISPR is profoundly difficult and expensive, and gets more difficult and expensive the more chromosomes are at play. Modifying mosquitos is much easier, and with the short generations (days or weeks instead of decades for humans) it’s much easier to get the genetic changes to stick and observe their efficacy. We might get around to modifying humans someday, but it will likely be centuries before it is available for anything besides fixing lethal anomalies (and even then, it’ll be a long time until that becomes consistently successful).