What was the idea behind simplification, given 行書 existed?
Simplified Chinese is much simpler than 行書, involving less strokes overall. Two reasons:


Many simplified characters were already derived from cursive forms (草書楷化). Examples include 書 → 书, 車 → 车, 興 → 兴. 草書 is much less strokes than 行書.
Simplification involved many different techniques, including removing entire radicals or portions of a character, like 習 → 习, or even reconstructing the character based on new radicals, like 響 → 响. Therefore there's many more ways to reduce strokes than relying on cursive forms alone.


So 行書 wasn't used because it's not simple enough. There was an attempt to go even simpler during the Cultural Revolution, but it failed due to being too radical.

YMMV of course; it's possible that 行書 is genuinely faster for you than Simplified, but on average less strokes = faster handwriting, and Simplified is generally faster to hand-write than 行書. See also: How much faster is it to write Simplified over Traditional?