"Why is God's sovereignty the key that upholds all His other attributes?"
God's power is above and beyond and beneath and around everything.
God's Word is clear that His will and purposes cannot be thwarted (see Psalm 103:19, 135:5-6; Job 42:1-2; Isaiah 14:24-27; et al.). But if God were not sovereign, we could have no confidence in who He is—not in our salvation nor in any promise revealed in Scripture. Consider: If God were not absolutely sovereign, outside circumstances could thwart His love for us, rendering it conditional (and thereby leaving us with no assurance of salvation).
Praise God that He "works out everything in conformity with the purpose of his will" (Ephesians 1:11)! God's knowledge, authority, power, and dominion are all extended to the smallest thing in the whole universe. His power is above and beyond and beneath and around everything.
You may be tempted to balk at this idea. You might even ask, "But what about free will?" Of course, you have free will, but your free will has no power over the sovereignty of God. God allows those who choose to reject His offer of forgiveness and eternal life through Jesus Christ to have what they want. He respects our choices. And this is also part of God's plan because God accomplishes all things according to the counsel of His will.
Thankfully, our God "does not change like shifting shadows" (James 1:17). Only God has the freedom to do all He wishes, so it is comforting to remember that His will is always holy, good, wise, just, faithful, and loving. Truly, there is no god like our God.