Rising costs to attend baseball games explain about 40% of the 5 million person decrease in attendance since 2015. baseballprospectus.com/news/article/5…
A sad finding in this article is that the effect of rising ticket prices doesn't reduce attendance enough to offset the additional revenue they get from charging their remaining fanbase more $$$.
... which means that it's in owners best interests to continue increasing ticket prices, further reducing MLB's attendance while juicing revenue. There's really no sign of that leveling off any time soon considering some teams charge >2x the median ticket price.
I try not to go into "baseball is dying" territory, but it's hard not to see future problems if the owners continue to extract more and more revenue from an ever-smaller, older audience.
When you combine the effect of ticket prices with the impact of tanking/seeing fewer and fewer competitive baseball games, we get to around two-thirds of MLB's total attendance decline. There are probably other factors but these are the biggest ones.
The good news is that they are both fixable; the bad news is that MLB owners have strong financial incentives not to fix them.
Here's the league average cost for a family of four to attend a baseball game and eat (FCI). Was relatively steady from 2012-2015; spiked >10%, $23, since then. 
The basic (simplified) math at play is that they increased the amount of money they can extract per fan by ~10%, but attendance only fell by ~5%. So long as revenue per fan rises faster than attendance falls, they can make more money by increasing costs.
