A couple of possible reasons. MICR printing has declined by a huge percentage over the last 20 years. I used to work at a bank in-plant where we printed MICR ink on offset and I witnessed the rapid decline there. If something isn't real popular, printer manufacturers aren't interested in its development. Second, and this is purely speculation, that MICR toner probably isn't compatible or isn't easy to make compatible with most types of developers and thus its use is very limited. Also, to print MICR correctly requires the use of a test reader to determine if it has the right density and is printed in the right position to pick up in actual readers properly. Printer manufacturers probably want to avoid this thing like the plague as there would be many variables, meaning too many complaints and service calls.
-Mark-