The problem is, that would limit my own option to make a version of the software and sell it under a more limited license in the future. Whomever I sell it to then has the right to go ahead and redistribute it, competing with me. Sure, my current, highly niche code already carries that risk, but the MIT license doesn’t stop me from releasing a modified version I may write that is more valuable as software, and then protecting that release with other licensing terms.