Why is there no vertical space between Bootstrap 4 rows

Bootstrap 5 (update 2021)

Bootstrap 5 has new gutter classes that are specifically designed to adjust the gutter for an entire row. Additionally, there is now the concept of vertical gutters to adjust the vertical spacing between rows and columns that wrap inside each row.

• use g-0 for no gutters
• use g-(1-5) to adjust horizontal & vertical gutters via spacing units
• use gy-* to adjust vertical gutters
• use gx-* to adjust horizontal gutters

Bootstrap 4 (original answer)

Rows no longer have margin-bottom in Bootstrap 4. Use the new spacing utils (mb-4).

See https://stackoverflow.com/a/42960338/171456.