The short answer is because the legislature decided this is what they want to happen. When you are arrested for DUI AND provide a breath or blood sample above .08% (.02% if a minor) or refuse a breath test there are two actions that can take place. The first is criminal charges and the second is an administrative action against your license.
Your license is considered a privilege and DOL can take action against your license for a number of statutory reasons, such as DUI arrest, unpaid tickets or too many tickets. A criminal conviction is not required for a license suspension. The State burden of proof and the standards of admissibility of evidence are much lower at a DOL hearing because it is a civil case not a criminal case.
DOL is also directed by law to suspend your license if you are convicted of DUI. This means you have to successfully defend both actions to avoid a license suspension. Because the deck is stacked against you in DUI cases, you will need an attorney's help if you want to have any realistic chance of winning.
The criminal case can result in a convction, jail, fines and a license revocation. You must attend court or a warrant will be issued for your arrest.
The DOL hearing is not mandatory and you are not required to attend. If you do not request a hearing, your license will be suspended. If you want to challenge the DOL suspension, you must request a hearing.
They are completely separate. Avoiding a license suspension on the criminal case doesn't prevent a license suspension through DOL and vice versa.
You need an experienced DUI attorney to help you with both matters.
Everything that has been said above is true. Remember that the outcome of the DOL hearing has no bearing on the criminal case, and in only the rarest of situations will the outcome of the criminal case have any impact on the DOL administrative action.