Tosafos asks that outside Eretz Yisroel where we sit in the succah on shemini atzeres, rain should be a siman k'lala even on shemini atzeres, so why acc. to r' yehoshua should we be mentioning rain? Tosafos answers that sitting in the succah on shemini atzeres is only "midrabonon bec. of safek" [Poras Yosef - meaning that since it is a safek we can be lenient to assume that elul was not me'ubar, but midrabonon they were mesaken to be concerned that maybe elul was me'ubar]. Since it is only a mitzah midrabonon to sit in the succah, it is not considered a siman k'lala to leave bec. of rain.
Another suggestion as to why rain on shemini atzeres is not considered a siman k'lala can be based on Tosafos in Succah. Tosafos says that we don't take lulav on s"a out of safek bec. it is degrading to yom tov to proclaim that it is still chol hamoed, but sitting in a succah is different since it is often pleasant to sit in a hut and not recognizable that it is for the mitzvah of succah. Rashi (ta'anis 2a) implies that the siman klala is the fact that we exit the succah due to mitzta'er. It would seem logical that when it begins to rain on s"a, even before you reach a stage of mitzta'er, you have to exit the succah to avoid degrading the yom tov. Therefore, when one leaves the succah on shemini atzeres, they leaving before the stage of mitzta'er to avoid the degrading yom tov. Since they are not leaving due to mitzata'er, the rain is not a siman klala.
