Though the Victorian era starts in 1836, it leaves 15 years of free space between Europa Universalis and Victoria games. An 1821 start would be a more natural transition and bring a new mechanic into Paradox games.
The first reason to want an 1821 start is that it allows a campaign to go straight on from EU4 without an awkward 15-year jump. Additionally, the Concert of Europe and post-Napoleonic balance of power already exists, so the global culture is quite similar.
The likely reason why this is not possible is the Americas, and not just because Paradox doesn't usually put much effort into South America. It's because 1821 is a date when something happens that Paradox has never traditionally liked - overextended empires are collapsing to rebels and financial problems. Spain and Portugal are losing their colonial empires after the ruin of the Napoleonic wars that they built during the time of EU4.
An interesting comparison can be made to the other periods of history Paradox doesn't model - the fall of Rome and the Han in late antiquity, and the cold war, which saw Britain and France unable to control their empires anymore after going bankrupt in World War 2.
Paradox traditionally love empire building, and hate empires collapsing and blowing apart into a load of smaller states. An 1821 start could change that, allowing for the first time the decolonization and the fall of empires to be a major mechanic. This could easily be transferred to a cold war mod with decolonization, showing why the European empires were doomed after 1945. It would be a final break from the obsession with conquest and growing empires ever bigger