Guessing it was a nightmare to change it in their codebase so they axed it. Also, increasing the limit would probably add no additional value to their business.
I'd argue the opposite. With them recently changing Direct Messages to account for 10,000 characters, they know what it takes to make the changes.
They know what it means to make the change, but it's entirely different to change something like direct messages, which go from one person to another, and the millions of tweets sent every day.
Do the Twitter APIs support direct messages? I imagine that the ecosystem of Twitter clients, libraries, site scrapers, and related database schemas have hardcoded the 140 char limit in many unexpected places. :)