Why do Oak leaves stay attached to the tree so long?

Answer 1

Marcescence is the retention of dead plant organs that normally are shed.
Both beech & oak retain their leaves til spring. It may be that other trees leaves are shed to reduce retention of insect eggs but oaks are very tannic so may have handled the problem of herbivory differently. Tannins reduce insect feeding. Leaf toughness also reduces feeding.
Oak trees have been dominant in Europe longer than any other tree species so have acquired more than 200 species of Lepidoptera. The feeding is primarily in the spring, & densest in May when oak bud break is. If the larvae hatch first the oak wins because the larvae can't chew the bud scales to get the leaves inside. If the buds break first the insects win by eating the tender new leaves. The larvae in turn are prey so can't delay their hatch to eat late opening oak leaves or they become easy prey. Oak leaves in June are fairly free of larvae.
Another thought is the leaves are frost protection in winter. The withered, non-nutritious leaves may deter deer from feeding on the tasty live twigs and buds under the dry leaf layer. This last relates to the leaves being retained best on the lowest branches. Retaining the leaves over the winter may allow leaf nutrient retrieval to last a bit longer in fall since the abscissa does not have to be completed til spring.
There are many theories and it may be a complex interweaving of factors that play a part.

Answer 2

because they can. a tree sheds it leaves to reduce surface area-exposure to the cold. a more cold-hardy tree will keep its leaves as long as it can.