Here's the long version of why:
Let's start with a potential Batman minseries: I'm going to compare the potential budget to that of Game of Thrones, which is $6 million per
episode. Let's say it's that, and the budget for the season is $72 million, and say that the advertising budget for the show is half. It would cost $108 million dollars to produce.
To put it in perspective, let's a 12 episode Batman miniseries grosses $14 million in ad revenue per episode (
which is would be a record, by the way). That would be only $168 million, but it would be successful enough with multiplier of 1.55 of its budget though.
Let's look at TDK in comparison. TDK grossed over $1 billion. The budget was $185 million and let's estimate the advertising budget is $100 million. To be generous, let's say the advertising budget is $115 and put it at $300 million. TDK made a profit of over $700 million, and a multiplier of 3.33 profits.
And the TL; DR version.
Batman tv show costs:
Per episode: $6 million
Per season: $72 million
Advertising: $36 million
total: $108 million
Batman tv show profit:
Per episode: $14 million
Season total: $168 million
Profit $60 million
Multiplier: 1.55
The Dark Knight Costs:
Budget: $185 million
Advertising: $115 million
Season total: $300 million
The Dark Knight Profit:
Total: $ 1 billion
Profit: $700 million
Multiplier: 3.33
The most successful
movie always trumps the most successful tv show in profits and with some franchises it's about maximizing your brand, and a tv show would never get the budget of a blockbuster movie because it wouldn't be justifiable.