Because there are People TOO laugh with. Diana had Steve with her the whole movie so did Aquaman with Mera same as this with Freddy. Sure MoS has Lois but they're were never together romping around bantering back and forth like the others were building that...um like-factor
. That's why people didn't buy them kissing amongst the rubble in MoS or them suddenly being a couple living together in BvS, people will buy anything if you sell it to them the right way. Why did people like Batflec in BvS? Cause he had Alfred to Talk too. Why did People like Superman more the Ultimate Cut of BvS? Cause Clark talked to people (whisper: He's Superman :whisper.). That's honestly my only problem with Snyder, He shoots long and sometimes has to leave the heart of his Movies on the cutting room floor cause I'm sure those parts are in a longer cut of MoS just like they were in BvS.
Sivana was way too Saturday Morning to truly take seriously. Plus Diana would have wrecked him magic or not which highlights the problem Marvel faced with it's extended universe, Where is everybody? If the Justice League Exists Why didn't they come down on Sivana like a Bad Cold? Now if you show Sivana Imprisoning the JL with Magic then all you have left IS Billy plus it would tie perfectly into the ending.