Why do current mountain bikes have such steep top tube? Comparing my Kona Blast 2001 vs 2019
Circa 2000 a Blast had 80mm travel and 26" wheels on 1.90" tires, compared to a modern Blast -27.5" wheels, 100mm travel, on 2.25" tires. The crown height on the modern Blast is significantly higher than its ancestors, necessarily raising the top tube height at the front. 

The seat tube with modern geometry has been shortened. Among other things such as handling changes, it helps with stand over height allowing a broader range of riders to fit the same frame.