There are only a few advantages a team can have during a season.
The most important is home field advantage. Every major league ball park is different. Team management needs to be aware of those ball park effects in order to maximize a team's home winning percentage since they play half of their total games at home.
For example, Petco Field in San Diego is a pitcher's park. Historically, Padre pitchers at home are 9.5% better than the league average. This season it has been 9%. The Padres are 38-22 at home (.543 winning percentage) which is better than overall record of 62-76 (.449 winning percentage). The reason is simple. It is a big ball park where fly ball pitchers do not get lit up. Pitchers don't have to be cute around the plate and pitch to contact to get outs. As a large ball park, it has wide outfield gaps where speed can turn doubles into triples. Home runs are rarer so a team needs to manufacture runs by other means.
Wrigley Field is considered a hitter's ball park. Historically, Wrigley gives a hitter a 3.5% advantage over the league ballpark average. However, this season it has jumped to 6.5%. Which is slightly strange considering the Cubs starting rotation has been pretty consistent and the wind has mostly been blowing in this year.
The Cubs have the worst home record in the majors at 27-44 (.380 winning percentage). At home, the Cubs have scored 297 runs but allowed 320, a 7.7 percent difference.
You need hitters to succeed in a hitter's ball park. But the current Cubs are woeful at the plate. The team is batting only .239 (14th in the NL), scored 533 runs (11th in the NL) and has a .303 OBP (13th in the NL).
When the new front office came to town, they stressed that they wanted patient players with high OBP. They have failed to fill a roster with those on base hitters which nullifies the impact of the team's 146 HRs (2nd in the NL). The new management team also wanted more left handed hitters on the roster. They have signed a ton of lanky left handed outfielders, but only 1, Nate Schierholtz, has hit 20 HRs. Anthony Rizzo has hit 21 in 136 games; Soriano is still 3rd on the club with 17 HRs in 93 games; and Dioner Navarro is 4th with 11 HRs in 74 games. The problem with overloading the team with left handed bats is that, as Billy Williams remarked a long time ago, Wrigley Field's power zone is left center field. Williams said he was successful at Wrigley because he had left field power. He said that left handed pull hitters do not do well at Wrigley based on the summer wind currents. This observation was born out in the mid-2000s playoff teams which were predominately right handed hitters.
In addition, the Cubs pitching staff needs to be mostly ground ball pitchers. Sinkerballers like Rick Reuschel were successful at Wrigley because it takes three ground ball singles to score a run rather than one deep fly ball. The Cubs pitching staff has no pure sinker ball pitchers. The Cubs pitchers can throw sliders, but those are not main pitches. The Cubs got caught up in power pitching - - - strike out types - - - who tend to give up the long ball more than control and sinker ball pitchers.
So the reason why the Cubs have played so poorly at Wrigley is simple: the team was constructed to fail at home. It does not have a pitching staff that gets ground balls outs. It does not have left center field power hitters in the line up. The team has no speed or high OBP for batting acumen to manufacture runs. The Cubs will be at a home disadvantage until the team roster changes to reflect the ball park features at Wrigley (which may change again when the new scoreboards are constructed which will alter the wind currents inside the field.)
