Two of Trumps children who are old enough to vote also did not register in time and thus they also fall into that same situation. 

Where there are important stringent deadlines such as this is is imperative that the candidates organizational machine understand this and get the message out in time. 

Just Fred wrote:

Hey, here's an idea for a functioning democratic society :  Let everyone who is of voting age participate in a primary or general election without any regard to party affiliation and standardize to procedure across the USA.

 
At the Federal level Presidential vote level you can certainly argue for that since our Founding Fathers  actually laid out some specifics about how the process SHOULD work, but at the primary level you are talking about two "business operations" (ie the Republican and Democratic Party) and how they choose to elect their nominee in the Primaries is wide open to the whims of the respective parties. Doubt that EITHER party would want to have outside rules laid on them and how their party functions (or NOT some might say). 

The Primary rules for each party are pretty much laid out well ahead of time for each perspective contestant to figure out the best way to win the nomination within the confines of the rules. I admit that EACH party has a lot of weird rules and regs, but that is their right and if people don't like them they are free to either change party affiliations or look for a third party candidate. 
 

All I'm saying is that rather than have each state set up rules and regs, simplify and standardize the whole thing.  we;ve got 'winner take all' primaries and 'proportion primaries', states and parties that set up registration deadlines 6 or more months in advance and states that allow you register the day of the primary. Then we have caucuses, conventions, delegates and super delegates, open primaries, closed primaries, delegates that reflect the will of their districts and those that don't, etc.

Why not just let the majority of voters who vote for a candidate decide who the candidate will be?  Actually, (and I can't believe I'm saying this, but) Donald Trump has a point here.

They will continue to construct things that they think is to their benefit. 

Right.  The hell with us unwashed masses who think we are living in some kind of democracy....... rule by and for the people.  Team politics is running us down the road to some new kind of plutocratic oligarchy.