The reasons I have heard are usually in favor of standing due to the status of the chosson and kalla as a king and queen. Another common reason I have heard is standing int he presence of the shchina due to a mitzva being performed. The reason I have heard against standing is that it is not a real custom but absorbed from the goyim.

I must say, this is far more common in the USA (and other countries?) - many chairs are put out. Most people sit, until the chosson and kalla come out and then many stand while some continue sitting. In Israel, very few chairs are usually put out, usually just for elderly grandparents (or others who cant really stand long enough for the chuppa) and most people stand for the entire duration of the chuppa.

Rav Yitzchak Zilbershtein, rav of Ramat Elchonon neighborhood of Bnei Braq, was recently asked about this. The petitioner mentioned that they had made aliya from abroad after they married and now they are marrying off a daughter who was born in Israel. The chosson was born abroad and doesn't live in Israel but is here learning in yeshiva and has been for a few years. Most of the family who will attend the wedding will be from abroad where the minhag is to sit at the chuppa. Because of that they had planned to arrange the chuppa with enough seating for the crowd, but perhaps that would be inappropriately changing the local minhag. What is the correct thing to do regarding the seating at the chuppa of this wedding?

Rav Zilbershetin responded paskening that the minhag is to stand at the chuppa and this was always done in Eretz Yisrael and in other countries, with only some communities in the USA changing from this custom, and not for good halachic reasons, so for this wedding in Israel they should stand for the chuppa even though the family will be coming from the USA. It is bad enough that they do not keep the proper minhag there, they shouldnt change it here as well, and the communities abroad should be influenced to adhere to the original custom and stand.

source: Hamechadesh

I found this interesting. I thought the reason so few chairs, if any, were set out at chuppas in Israel was just practical or just the way halls here do it due to there being not as much as space or not having enough chairs with them all being set in the hall for the meal and other reasons like that. I did not realize it was an actual minhag dictating this...