A while a go there was commentary about how a large number official duties by the Royal House took place within a set area of the country. I assume Emmen is out of that area? Any that may be partly a reason it was chosen?
In terms of provinces you could divide the country in 4 areas of 3 provinces each (number of inhabitants in millions between brackets):
- North: provinces of Friesland (.66), Groningen (.60) and Drenthe (.50) --> 1.76 million inhabitants
- East/Central: Flevoland (.44), Overijssel (1.18), Gelderland (2.13) --> 3.75 million inhabitants
- West/Central: Noord-Holland (2.95), Zuid-Holland (3.80), Utrecht (1.39) --> 8.14 million inhabitants
- South: Zeeland (.39), Noord-Brabant (2.63) and Limburg (1.13) --> 4.15 million inhabitants
The majority of the population lives in the 'West/Central' area, which includes the 4 largest cities: Amsterdam, Rotterdam, Den Haag (The Hague) and Utrecht and many smaller cities. The far majority of royal events (about 2/3rds IIRC) also takes place here.
For Koningsdag to me it makes sense to ensure that each province is visited over the years. So far, the king concentrated on the larger cities that are concentrated in the west/central part and Noord-Brabant (5th, 7th and 9th; in the South) with the exception of Groningen (6th largest) in the north.
The cities/towns visited (between brackets their ranking in size of municipality and number of inhabitants in thousands)
2014 (postponed due to the inauguration in 2013): De Rijp (small), Amstelveen (top 40, 94) --> Noord Holland (West/Central)
2015: Dordrecht (25th, 122) --> Zuid Holland (West/Central)
2016: Zwolle (19th, 132) --> Overijssel (East/Central)
2017: Tilburg (7th, 228) --> Noord-Brabant (South)
2018: Groningen (6th, 238) --> Groningen (North)
2019: Amersfoort (16h, 161) --> Utrecht (West/Central)
2021: Eindhoven (5th, 244) --> Noord-Brabant (South)
2022: Maastricht (23rd, 123) --> Limburg (South)
2023: Rotterdam (2nd, 664) --> Zuid-Holland (West/Central)
2024: Emmen (29th, 109) --> Drenthe (North)
So, so far the king visited the West/Central part 4 times, the South 3 times and the East/Central and North both 1 time. In terms of provinces both Zuid-Holland and Noord-Brabant were visited twice, while Gelderland (the 4th largest province in terms of inhabitants) has still not been visited by the king on King's day. Both Nijmegen (10th) and Arnhem (12th) would be excellent options for a future king's day.
Scroll down on wikipedia for a map with 'dots' for each city he visted.