The designers thought it was a balance issue.

See the following Tweet from Mike Mearls, the rules designer.

Question: Is there a specific reason to explain why you can only cast a cantrip after a bonus action spell? Curious.

 

Mearls' Answer: It was a perceived balance issue, mainly to prevent massive volleys of spells; not sure we actually needed it.

The designers thought it was a balance issue.

See the following Tweet from Mike Mearls, the rules designer.

Question: Is there a specific reason to explain why you can only cast a cantrip after a bonus action spell? Curious.

Mearls' Answer: It was a perceived balance issue, mainly to prevent massive volleys of spells; not sure we actually needed it.