Well, people tend to focus all of their EVs into 2 or 3 stats mainly because of the fact that you can't have any more than 510 EVs total. There is a limit there, and if you divide up your EVs evenly among all of the stats, you are not going to get a significant gain in any of your stats. Therefore, it is a good idea to pick out the most important stats for each of your Pokémon, according to their moveset, stats, personalities, etc... and EV train those stats. Because there is a limit of how many EVs you can give your Pokémon, you have to be careful, and you don't want to waste them. Every 4 EVs in a certain stat will equal 1 more point in that particular stat. If there wasn't the 510 total EV limit, then you could go ahead and give your Pokémon EVs in every stat, but since the limit exists, the best thing is to give EVs to the most important stats for that Pokémon. This is the main reason why most of the sweeper Pokémon in this generation of games are either physical sweepers or special sweepers, you don't see many Pokémon that are special and physical sweepers anymore, because it usually isn't beneficial to be using up your EVs on attack and special attack, as there are other stats that need to be worked with.