They chose a cheap, reliable, and easy to obtain lump. Period.
The rover K cannot be sold in america currently, but potentially it could be certified and sold here. Financially speaking it is not prudent to get it certified, plus it is getting long in the tooth and will probably be replaced at some point in the future. We already have plenty of engines available that suit the elise, the toyota only being one of the bunch. Honda makes a dandy fine engine, as does mazda (renesis), but for political and technical reasons they were not chosen. I dont know the specifics but I can make plenty of guesses...
In the end we have a tradeoff engine. The toyota engine produces an amazing amount of power given its weight and size, but its upper limits are low due to the manufacturing tolerances, compression ratio, and complexity. That being said its base format has lots of power through a large range making the elise very easy to drive. While I would rather have more power, a narrow powerband would not sell well, and it could very well increase our insurance rates from people not able to control an abrupt power change. (Ala the s2000, remember the weight difference here)
Do I wish another engine was selected? Yeah kinda. I would have loved to see a rotary, but I cant figure out where the 20gal tank would sit. The honda? Yeah again but politics play a big part. So we have the toyota engine. If lotus puts it in play through the world and not just here, we will see tons of mods available for it. If lotus does not fix the internal issues themselves, someone else WILL, and hopefully they wont be asking for 2100 dollars for a muffler (ala porsche) that only increases sound and has no hp change.
My guess? 3 or years from now we will see the elise sport 250. Lotus isnt stupid. FIrst 2 years the car sells out 9months-1yr ahead of production. Third year the sales dip a bit as people get disillusioned about driving the car all the time and start selling, new car sales slow a bit. Time to do some racing, and release a club sport edition. Roll cage everything bolted in, and some engine "enhancements" ready for ya. 220hp. Another year and they get serious with an "off road" version with 250hp.
They can keep the car going for years and years making good profits with a minimum of change (ie $$$) invested in the current platform. Then when they finally need to, release its replacement. 5 or so years from now.
Oh and have an exige somewhere in there too.