Well, probably because they don't want the ability to scale with AS.
When an ability scales with As there are 2 stages:
- It's strong enough to build AS on hit, then an AS build is introduced and abused.
- The on-hit damage is no longer high enough to justify a build, and people start to rank that ability last and work around other aspect of his kit.
Take a look at what happened to Skirmisher's Sabre and the Devourer enchant.
On hit?
well, Warwick with Botrk and Skirmisher OFC.
Or Yi, xin Zhao, or any other champion who can abuse the damage by attacking multiple times.
On the other hand, Applying a DoT means that there is both a window of time where the damage is being applied (thus allowing the player to turn around and escape) , and it can also be rewarding without the need to build AS.
In fact, Skirmisher Sabre on the PBE no longer deals on-hit damage per attack, but applies a DoT on hit.
TL;DR DoT on hit are healthier for the champion's kit because they cannot abuse attack speed items.