Why professional is considered to be the default difficulty?

Master has no chance to not be beaten by same strat (or much easier one) on Pro. Difficulties are different. Every second matters, thats a speedrunning, isn't it? So how can you compare two basically not equal things.

It should be splitted or Master will simply die same as some people's interest in getting "a quick world record" with harder conditions. Like repeating someone's strat on the same difficulty is not the same quick world record...

Hitman WoA: Category Extensions

As a trial, difficulty has been changed from a subcategory to a variable. This effectively merges the pro and master category extension runs onto one leaderboard. 

The difficulty is still indicated on the leaderboard, and if you wish to submit both a professional and master run, both runs will show up.

The reasoning behind this change: There simply is not as much interest in master difficulty category extensions to really justify existence as a separate category. 

With the exception of master/ default loadout, which is relatively popular. However, merging the leaderboard will help the master/ default loadout runs in most cases, as the master/ default runs have become more optimized than the pro/ default loadout runs in several levels.

If the change proves unpopular, it is easy to revert.

There are currently no plans to apply this difficulty merge to the main leaderboard (https://www.speedrun.com/hitman_3), as there is a higher level of interest in playing Master there.