Fouls Balls Led to Walks
The foul strike rule was first adopted by the National League in 1901 as a response to some players (most notoriously Roy Thomas) developing the ability to foul of pitch after pitch to force a walk. Rulesmakers thought that this upset the balance between hitting and pitching. It was also disruptive because umpires normally had only two game balls at a time, and balls fouled off into the stands had to be retrieved from spectators. The foul strike rule was adopted to penalize players for hitting too many fouls.
Leagues Differ
The American League did not adopt the foul strike league immediately, and the rules difference probably contributed to higher offense in the AL than NL in 1901 and 1902. The AL adopted the rule as part of the NL/AL peace agreement in the 1902-3 off-season. The adoption of the foul strike rule has been suggested as a possible contributor to decreased scoring in the Deadball Era.