This is most likely because you could now access areas much earlier in Black/White 2. If the Pokemon were at the same level as in Black if you were to reach Lacunosa town, then they would be too overpowered and make the game to easy. It would also be harder to catch them. For this reason it is most likely so that Game Freak diminished their levels to a fitting amount to match the game's update. However this was evened-up a little since that the Pokemon located in the early routes of Black/White have now witnessed a major increase in level since you can only access those areas later in the game,
To keep it short, the main reasons why their level was decreased was to not make the Pokemon have a too high level when you reached a certain stage in the game. However this only seems to have an effect when you reach the 6-8 gyms
