In part because the carmakers were probably unsure about its long term performance with a totally new technology, but the main reason was other.
It just happens that the environmental emissions of a car maker fleet was calculated as the average of emissions from all their models in the current market.
Those tiny, squared EV that looked like toys were built with the only purpose to decrease the brand average value. Thus they could have models contaminating more in the upper categories. (Plus painting the brand in green for PR purposes).
All of this changed after Tesla started to show its claws and to be taken seriously.