why bip 34 uses two step on MASF(Miner Activated Soft Fork)?
The two thresholds enable two different rules.

If more than 750 of the last 1000 blocks are version 2, the network starts enforcing BIP34 - at this point, blocks that claim to be version 2 but do not comply with the requirements outlined in BIP34 are rejected. Version 1 blocks continue to be accepted.

Once more than 950 of the last 1000 blocks are valid version 2 blocks, all new version 1 blocks are deemed invalid.

This allows for a mostly graceful transition, and gives miners time to upgrade their systems in a rolling manner as opposed to requiring everyone to update at the same moment.