I think the main reason they did not make the transition to (partially) electric cars earlier is that they did not want to cannibalize their business with non-electric cars. Another point is the still-missing charging infrastructure and the (to my limited knowledge) lack of some universal standard for charging stations. The final point is that today potential buyers of electric cars still have many good alternatives that are as much or even more ecological than all-electric cars. If you look e.g. at the new models from Volkswagen or BMW, many of the smaller ones use 4 liters of gasoline / 100 km or less (i.e. > 71 MPG), which is pretty hard to beat in terms of eco-friendliness and efficiency considering that electricity is still much more expensive than gasoline in terms of energy / price.
One driving factor in the transition to partially electric cars might be that there are not many other possibilities left to further increase the efficiency of conventional engines. In fact, most of the increase in efficiency that we have seen in the last decade was not due to better thermodynamic design of the motor block but better control of e.g. the fuel injection through more advanced sensors and electronics. As this optimization path seems to have reached is maximum potential it probably makes a lot of sense to use electric motors to e.g. recover braking energy, which should shave off another few percent in fuel consumption (Toyota already proved the feasibility of this very successfully with their Prius series).