economic reasons, Lotus had been playing around with various different engines long before they decided to change over to the toyota lump. The reason they didnt do it earlier was simply due to money reasons, they didnt want to spend anymore money when they had what was perceived as being a 'good compromise' between cost and workability
besides the very poor design of the k-series engine gaurenteed work for their mechanics etc...
It is a very poweful lump in the right state of tune... would i have one? definately not for the reasons i have previously stated. The jump to toyota was probably brought about sooner due to the collapse of the rover, but it was going to be done regardless of that situation due to the drop in elise and exige sales on the back of poor publicity regarding reliability and mileage problems etc...