There are essentially five driving forces for MMORPG players. These games, by their very nature have numerous (or at least multiple) activities and each player is motivated by the following:

1. Progression - Experience to level, or obtaining gear at Endgame.
2. Rewards - Gear, currency, notoriety or any other status-symbol or in-game possession.
3. Fun Factors - Is the activity just plain fun to do, or awe-inspiring in some way?
4. Competition - Many players are keen to compete and compare their performance with other players / groups / guilds / factions.
5. Social Interaction - At the core of any multiplayer RPG is the basic social satisfaction from hanging out with your friends & guildmates, teaming up in a cooperative way and meeting / interacting with new people. Human desire to be accepted as part of a group should not be overlooked.

Each of these can motivate players in different directions. For instance, an activity could be fun, but the players may feel discouraged if it doesn't provide experience. It's great to be social, but if solo'ing is the fastest way to level, players will shun teaming up.

The first two motivations can be considered incentives in their purest form, in a way they are both rewards although it's important to separate them because one will motivate a certain type of player whereas the other may not.

In an interview with Ten Ton Hammer, Mark Jacobs repeated an established adage in MMORPGs:

I'd call this more truthiness than truism, because it's missing an important element--

Add Convenience

Convenience is the sixth motivator and it's the trump card. You can take any combination of the above and mix in convenience and whatever the activity is, it will go over in a big way. One example is if quests are trackable on a map, most players will naturally complete the quests that are closest rather than any sort of story-progression.

Players will always find the shortcuts.

For Warhammer, Scenarios match multiple categories, they're certainly competitive and fun and if the queues are short the experience is good. But above all, they're super-convenient.

Mythic may try to nerf Scenarios, or more likely boost the incentives for other activities, but the fact of the matter is that they need to boost all of the motivators, not just exp and rewards if they hope to make their game more well rounded and get players deeper into RvR and Public Quests.