Some considerations about the bo3 choice in this year's UPL. Below reasons are the main arguments for carrying out UPL this way.

-Willingness to play, the logical consistency argument
The Ubers playerbase, and the tournament playerbase on Smogon as a whole participates in numerous tournaments over the year. Many of these use bo3 as their standard, and are still carried out by weekly rounds. Examples include Seasonal circuits and Smogon Grand Slam. We therefore are lead believe the playerbase are thrilled to complete matches, and that they can spend the extra effort on making sure to uphold quality in these, which is reasonable considering evidence from other tournaments with bo3 fights.

-Quality of results and play, the argument for the competitive dimension
Evidenced in a bo1 one thing has always been consistently prevalent, inconsistent results. Results do unfortunately not always reflect the quality of players, as we are cursed to play Pokémon. Over a larger sample size, it is more likely the better player wins and thus the better team. Similarly, it is more likely a better player, or a player with a better team environment will truly have their quality reflected in their results. Now, it is not likely that everyone has the same amount of time to build, but there is plenty of time before the start of the tournament. This issue will, however, be adressed better in the next point.

-Teamwork, the argument for quality and inclusiveness of team-oriented activity
While it is a worn-out cliché, a good team in any setting is characterized by something along the lines of "the sum of the whole becoming bigger than its individual parts". A good team environment can and will produce better teams faster, help out those who lack the time during a certain week, and in general compensate for shortcomings. This is further bolstered by a bo3-setting: now good teamwork will truly be required to meet the higher demands for team creation, people hiding away on their own will either display worse results or be detrimental to creating a strong team environment.

-Past generations, the argument for bolstering activity of dead tiers
Many players have expressed their love for older tiers. Honestly, Ubers might be the tier that loves their old metagames the most and UPL is the best chance to play them on a high level. It is such a shame to only give these players 5 measly games to show their new techs.

Counterarguments and refutal of these:

-"The inconsistency of a bo1 is void by the team tournament effectively being a bo8"
Nothing is ever completely certain but a larger sample size always helps reduce this uncertainty. Pokémon is the game we play and we should be prepared to face the large uncertainty by measuring larger sample sizes to acquire precision in our judgments, both about players and Pokémon.

-"We won't have time to build 3 new teams every week"
While the second, and mostly, the third point refutes this, it is also worth adding that a bo3-setting means that recycling teams is not necessarily even a bad thing. Common tactics in bo3s include swapping certain spreads, sets or "hidden" Pokémon (think Arceus-formes). Being able to build a team that lasts while the novelty value of it is gone is a seemingly forgotten aspect of being a strong player. So no, you don't have to build 3 new teams each week.

-"Focuses less on teamwork"
There is no evidence this should be the case. If anything, teamwork is now more important.