There are several possible explanations (listed by order what i think is most likely):
- dell is close to intel (thats a known fact) and got a very good deal on those processors and thus tries to sell them
- providing core i5/i7 to such a small formfactor required more time to get it ready...
There are ulv processers for core i5/i7:
http://de.wikipedia.org/wiki/Liste_der_Intel-Core-i-Prozessoren#Mobil
Though thier TDP (termal design power=max watt they can draw) is 18, so 8 more than the 10 of the core 2 duo SU7300, the chipset the core i5/i7 uses draws only 3,5 watt max versus...