The Japanese Emperor's role isn't about being a ruler. It is about being a prayer who attends ceremonies for the good of Japan, to wish for the people's welfare with all the humility of having the privilege.
As such it isn't often that they are the center stage to a celebration. This ascension ceremony this year is an exception, and such festival of being in the spotlight, is one of very few events in the life.
So this is a very rare occasion for the open-topped limousine to be used in a public event.
Take a good look at the 10 minute drive of the Toyota Century Royal "Sumeragi 10" as this drophead Century may never appear in front of a video camera for another decade or more.
The last time the open limousine (then a Rolls Royce Corniche Drophead) was used in a parade was when the Emperor as price had the wedding with princess Masako.