I've played about 3 sessions and I DM'd my own session a few weeks ago (playing 5e edition), so i'm pretty new to all this. I've been reading the rules in the handbook and on these forums and I came across the following rule about multiple spells in a single turn (bonus action + action, PHB 202) which basically is the following: during a turn you can do:

• Cantrip (action) + Spell (bonus action)
• Spell (bonus action) + Cantrip (action)
• Cantrip (action) + Cantrip (bonus action) I believe this follows out of the PHB on page 202.

My question is: WHY is the action restricted to a cantrip? Why isn't it allowed to do the following?:

• Spell (action) + spell (bonus action)
• Spell (action) + cantrip (bonus action)
• Cantrip (bonus action) + spell (action)

My thinking is: most rules are there for a reason and when you follow the PHB-rules everything should be balanced. So the question is would it be OP to let my players do the above, or does it mess up balance between classes? When they cast 2 regular spells in a turn they DO have to burn 2 of their spell slots... and looking through the spell list I can't really find any super powerful combinations you can do with this.