It's more about Home Runs replacing Singles than Triples declining.
Distribution of 1B, 2B, 3B, HR as percent of Hits and Total Bases 1903-2017. Sunday, March 11, 2018
Lately some attention has been given to the reduction in total triples in recent years. This post puts that into a broader context. See graphs below.
2017 had the lowest percent of Hits (64%) and Total Bases (38%) that were Singles.
2017 had the highest percent of Hits (14.5%) and Total Bases (34.6%) that were Home Runs.
Doubles have been pretty constant since 1997. For each of the previous ten years (2008-2017) about 20% of Hits and the percent of Total Base has trended down from 26% to 24%.
Doubles Percent of Hits:
low 12.5% in 1907; Singles peaked at 81.3%, HR lowest: 1.24%
high 20.5% in 2008; reached 20% for first time in 2001
Doubles Percent of TB:
low 19.7% in 1955
high 26.6% in 1931