Originally, Yizkor was said on Yom Kippur and only later was it extended to Shelosh Regalim. Why was it not said on Rosh Hashanah which is Yom Hazikkoron? It would seem more appropriate to say it then rather than on Shelosh Regalim, as it is contrary to the spirit of 'veSomachta beChagecha'.
Divrei Torah Volume 3 #61 mentions a reason that I have heard - which he says is inaccurate. The reason he mentions is that Yom Kippur is like the last day of Rosh HaShana and we always say it on the last days of the Yom Tov. He says that is not accurate as they are separate Yomim Tovim. He then says that on Rosh HaShana we are only supposed to think of live things כי בר״ה יחשבו רק מעניני חיים and through that we will be written to life.