Scrolling works in vim by default in gnome-terminal but not in mrxvt

[Countable Infinity <countableinfinity@gmail.com>]

Experiment 1
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1. Open gnome-terminal
2. Run vim
3. Run command :help in vim
4. Use scroll wheel of the mouse to scroll
5. Scrolling happens successfully

Experiment 2
===========
1. Open mrxvt
2. Run vim
3. Run command :help in vim
4. Use scroll wheel of the mouse to scroll
5. Scrolling does NOT happen successfully
6. Rn command :set mouse=a in vim
7. Scrolling happens successfully

Could you please tell me why this difference occurs?