^ Because they are invited speakers?

There are many more invited speakers, but I base my prediction on an analysis of the list of front runners compiled by this poll:

https://vote.pollcode.com/44839318

Peter Scholze is the strongest candidate in years and will definitely get it. The question is what other 3 people will also win.

Geordie Williamson made major contributions (as a bulider - opening up new venues) in a field that's not too crowded. The gap between him and teh next best in his field is significant. He is an Australian, an advantage in the spirit of spreading teh medals beyond Europe and America.

Sophie Morel would have been a strong contender but there are 2 issues. First her field is close to Peter Scholze's. It almost never happened that 2 medals (in teh same year) go to 2 people in the same field. ALso, she is a student of Laumon, whose two other students B. C. Ngo and Laurent Lefargue have already won.

Alessio Figalli and Simon Brendle will not win because they publish too many papers. Figalli has over 100 papers in less than a decade. This actually diluted their impact.

Maryna Viazovska solve the sphere packing problem in dim 8 and 24. Certainly an outstanding achievement but the problems are isolated and their solutions don't have broader impacts. Also, Viazovska is young and will have another chance in 4 years.

Contrary to public belief, Fernando Coda Marques (a Brazilian at home coming) will not win. Because Atur Avila (another Brazilian) has just won in 2014. And because Marques' most important papers were all coauthored with a senior mathematician (who is over 40 and not eligible).

By elimination, this leaves Ciprian Manolescu and Wei Zhang as the two remaining strong contenders. Both look good. Manolescu's specialty is geometry, significantly different from Scholze's and Williamson's. He has contributed to both solving an outstanding problem and building a new field. Wei Zhang is the leader of a young generation of Chinese mathematicians. His paper on higher Gross-Zagier formula is a major breakthrough. His coauthor Z. Yun is junior to him and this is Zhang's last chance. This is also Manolescu's last chance.

That sums up what will happen in Rio in 2018. Thanks for reading.