Because during Pep's Barcelona the midfield was the creative center of the team. And people, when they say Barca DNA, or Barca style, refer to Guardiola nowadays.
1) Pep system, the first part (with Messi RW, Eto'o CF, and Henry LW), had the midfield at it's core. The midfielders (Xaviesta + Busi) were in charge of constructing the play. The passing network from midfield (1-2s, first touch passing, pass and move, triangles etc.) had the purpose of a) keeping possession, and b) playing the front trio only when there was an opportunity for a great pass, not just anyhow, but playing them in space, in dangerous positions. That is why the playmaking came from our midfielders. Iniesta and Xavi had very high assists or pre-assists numbers. Either the final pass, or the pass before the final pass came from them. Basically our midfield passed and passed and passed and moved the ball until it found our attacking players in good positions (and our attacking players made runs, or switched positions, trying to make themselves available for a pass, while also staying in their place to maintain width and stretch the opposition). Very important, when our attackers where not in a position to receive a great pass, the midfield would not release the ball, but would re-do the cycle of passing until that space was created.
2) Lucho system. This is where it changes a lot. We do not keep MSN as only targets in the last 3rd of the pitch, as attacking players you activate from midfield playmaking. We use MSN as targets from all areas, including direct balls from defence to our trio, consiering that they can make the difference individually, even in positions where they have to overturn a numerical advantage in favour of the defending team. The main job of the midfield is to play the ball to Messi, Neymar and Suarez, and not just in good positions, but everytime is possible. This is why the media has been writing about Iniesta in the last years being more of a water carrier in this team. And this is why Iniesta was at his best last season when Messi didn't play vs Madrid, on Bernabeu, probably his best game in the last 2-3 years. Because he was allowed to exercise his creative role more, in the absence of the team's no1 creative force - Messi. When all of Messi, Neymar, and Suarez are on the field, they are used not only as great attacking players, but as great playmakers too. Because they are, and because it is handy. The result for this, was that MSN, having to do both the creation part, and the attacking (finishing) part, is draining them so much in possession that they either get too tired late in the season (2015-16 problem), or they do very little off the ball in order to rest during the game. And if we allow them to rest off the ball, and not make them defend, we need players who are less playmakers, and more hard workers. Hence why we signed Rakitic over Kroos.
