In short, there are two reasons for the migration away from DIL
1) Shorter leads inside the package (Michael, I believe, discussed this here abt 2 weeks ago) to gain less propagation delay and voltage drop.
2) The industry want smaller packages (have you seen a 10 year old cellphone?)
also)Some packages such as PLCC are developed as a compromise between the above reasons and a desire for socketing certain components. With ISP/IAP becoming more and more common, these will, probably, fall by the wayside in a few years.

Erik