The Senate choose not to remove him from office is more accurate. But you'll have to go with the only response that mentions the Senate.
Many of the people who have come to Mexico City have ended up living in "surrounding" areas, since life in the city itself is very fast paced and dense, meaning that many people wish to enjoy the benefits of the city without being there directly.
The answer is: Political bosses expanded their power by helping immigrants in return for their loyalty.
At that time, many of the political bosses realized that if they somehow can help the immigrants obtain their citizenship, the immigrants will become much more likely to return their favor.
This mean that they can influence the immigrants to vote for them during the election in exchange for the citizenship. This help the political bosses expanded their power.
