Is it the fact that HBD is limited to 10% of HIVE?

Yes, this is the big one. Currently the proposal is to increase it to 30% with the next hard fork, but that's still a very conservative value.

Could you point me to an article if possible?

This is a good starting point, and I've added some comments to attempt to clarify the key protection:
@taskmaster4450/hbd-defenses-how-hbd-is-different-from-ust