Why do secured bonds have less default risk than unsecured bonds? Secured bonds are backed by specific assets of an issuer while Unsecured bonds are backed by all assets of the issuer - so shouldn't Secure Bonds have more default risk?
Unsecured bonds are backed by all assets of the issuer
No, they are not secured by any specific assets (hence "unsecured").
With a secured bond, in the event of default, the bondholders automatically take ownership of the assets used to secure the loan and can sell them to pay off the remaining debt. The issuer often must ensure that the value of the assets is enough to completely pay off ("cover") the bonds, and is restricted from selling them (similar to how you can't sell a house or car that you have a loan on without paying off the loan first).
With unsecured bonds, there are no assets used to collateralize the loan. If the bond issuer defaults (i.e. cannot make a coupon payment), some manner of legal action (typically bankruptcy) must be taken to recover any part of the bond. That recourse may come from a sale of remaining assets, but there are no specific assets pledged to the debt.
Secured loans often have lower coupon payments, which is a tradeoff for increased safety for bondholders and restricted use of assets for bond issuers.
Since the recovery rate is higher for secured bonds, there is less overall default risk.
Secured bonds are specifically backed by a particular collateral. Unsecured bonds are "secured" by the debtor's assets, but a secured bond is also secured by the debtor's assets, as well as the collateral. If the collateral isn't enough to pay off a secured bond, the debtor doesn't get to say "Too bad, that's all you get" ; the creditor can claim the collateral and go after the rest of the debtor's assets.
 There is such thing as a "no recourse" loan, where the creditor can't go after other assets, but generally speaking, bonds aren't no-recourse. Although the corporation as a whole is a limitation on recourse, as it allows stock holders to shield their personal assets.