Practical reasons - people ramped up difficulty on testnet1-2 and then stopped mining.
Waiting several hours for a single block to confirm interfered with the purpose of testnet. Hence the reset rule.
Not that reset, I mean the new genesis blocks.
Testnet is still difficult to mine as long the 20 minute rule is upheld.
From that perspective, the value can be derived from the stakes miners have in difficulty not being reset as it would massively devalue the coins they've mined thus far.
The "risk" of reset is immense, because all miners would have to tightly cooperate (not to mention external factors such as ddos attacks).