It's to discourage hoarding 500 energy (starting 3-4 days before ladder starts). By splitting the Bazaar into 2 weeks, players that want to hoard 500 energy for the ladder will be hard pressed to exchange for stuff they want in week 1. This goes along with forcing players to farm Normal/Heroic difficulty in Week 1 to get the 100% achievement if these players also want to exclusively do Master-only in Week 2 ladder.
Though currently, it is still possible to exchange all the limited items (other than gems) in week 1 AND still store 500 energy, because the 30 and 50% chests gives quite a bit.. You just won't have any tokens left for any monster/rune/heirloom tickets. Farming 100 Normals for the 100 Dark Barons gives very little tokens.
I think this reason is highly possible. IMHO, if this is the case, the devs should just reduce energy cap instead of punishing players base. Hoarding energy is not really needed despite some players taking advantage from it to progress further (without spending). Removal of ability to hoard energy will only put every players at the same start point of each event ladder which has nothing wrong. And it's better than limiting game play of the whole players base by dividing bazaar into 2 parts.
The funny thing is, if you reduce the energy cap, the group of players that will complain the loudest is actually the F2P players. Because these are the people that say that hoarding energy is the only way to get high on the ladder, and if you lower the cap (and it was lowered from 999 to 500 a while ago too), it reduces the effectiveness of F2P to be able to compete on the ladder against whales.
Mainly, the point is that if every player starts with no energy stored, then the top players will always be the ones that spends more gems to buy energy, because each Master event run gives a fairly fixed amount of ladder points. If everyone can clear Master, then it's pretty clear that if you can do 100 events vs the guy that can only do 90, the guy that does 100 is going to win, simply because he has a bigger lunch/dinner energy bonus, and a water mill that gives more energy per hour, even if both didn't spend money for the event, but one has higher prestige level.
After all, the current ladder doesn't care if you can clear the Master event in 14 rounds or if it takes someone else 29 rounds, they will both give the same amount of ladder points.