Why do current mountain bikes have such steep top tube? Comparing my Kona Blast 2001 vs 2019 I am very curious about the geometries of mountain bikes nowadays (or probably since a while back).

They all seem to have very tilted top tube .. compare my Kona Blast 2001 vs 2019




Kona Blast 2001





Kona Blast 2019

For the newer bike that fits well when riding .. have tendency to be hitting my crouch when I dismount somehow.
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.