I would re-number this era as 1893-1919. Nothing happened in 1900 that was significant (rules changes). The mound was pushed back to 60'6" in 1893 and Babe Ruth had his first big year in 1920. Actually, Ruth had 29 HRs in 1919 but Fenway was huge back then. Ruth only hit 9 HRs at home but 20 on the road. given a neutral ballpark it is likely Ruth would have hit 35-45 HRs in 1919.
The biggest change and the reason the era is usually split at 1900 is the establishment of the true two league system and the World Series. There also was a very major
rule change for the NL in 1901 (and 2 years later in the AL) with the addition of the foul strike rule. It dramatically altered the way the game was played. The 1900 mark is somewhat superficial but is usually the commonly referred to break point for the beginning of the Modern Era.