EVE Online limits number of players in Jita hoping to reduce lag.

In a series of hotfixes, EVE Online has created player caps in the area of Jita to help reduce the amount of lag. Unpopular as this sounds, it should also be temporary. According to the forums:

"During the downtime today, Sunday 21st September, we deployed a new feature to Tranquility. The Stargate will now check how many players there are in the system and deny access if you attempt to jump into it when too many players are present. In addition to this you will not be able to login on a character if the system has too many players. We have also increased the amount of GMs this evening and if you are unable to login, please press the ESC button and submit a petition in the Stuck category.

This is a temporary solution to this issue, we are working on a permanent feature which has the highest priority and will be implemented as soon as possible."

Thus illustrating one of the issues with single-server populations, EVE appears to be sagging under its own weight. Growth is good, but they need to be able to handle it as well. The CCP team is working on this issue so anticipate an update shortly.