Why do people who come to Israel for the shalosh regalim keep two days if there is no mention of people who were Oleh regel keeping two days during the first/second temple?
Obviously -- so long as the Sanhedrin was declaring a new month and sending messengers -- which continued well past the destruction of the Second Temple -- anyone who happened to be in Israel on yomtov would keep one day, because they knew.
(And yes, there are those who opine "follow the locals as of today", which also means an Israeli on a one-week business trip to the US keeps two days. But let's assume, for the sake of this question, that we follow the Shulchan Aruch, R' Moshe Feinstein, and R' Shlomo Zalman Auerbach -- that you are defined by your "home" vis-a-vis how many days to keep.)
When we switched to a fixed calendar (let's assume around the year 500) and the Talmud ordered diaspora Jews to continue keeping two days, some of the parameters may have thus changed in that decree, compared to what people had been doing previously. It appears the decree was: "Diaspora Jews keep two days." Some have suggested this was a way of gently pushing folks to move to Israel, or that Jews outside of Israel needed more of a spiritual lift. We already had treatments of "local practices", for which you are defined by your home; so how-many-days-should-I-keep fell into this category as well.
(Just to make it even more fun: when they relied on messengers, who kept two days? People who lived beyond the range of the messengers. Rambam says this is estimated by the minimalist borders in Gittin Ch. 1, and therefore those are the places that keep one day of yomtov. Ritva, if I'm not mistaken, says that when the enactment changed from "keep two days if you didn't get a messenger" to "keep two days outside of Israel", they used the maximalist definition of "Israel", i.e. the borders found at the end of Bamidbar. Even though that didn't align perfectly with what the practice had been in the messenger era.)