Two primary reasons:
- Difficulty/ease of copying: What Samsung has [manufacturing expertise and plants for chips, displays, DRAM, flash memory, etc.], Apple can't cheaply or easily copy. What Apple has [product design, software features, user interface, etc.], Samsung can cheaply and easily copy. What is noteworthy here is that Apple might have spent millions to come up with a particular design, and it's as easy for Samsung [and anyone else] to copy it as it is to say "copy it".
- Difficulty of proving infringement: Whether or not two designs are similar depends on human judgment. We do not have an objective way to evaluate the degree of similarity of two designs. So even if it is proved that Samsung did copy Apple's designs, it is easy for Samsung to modify them only so much that they're sufficiently different to address legal hurdles, although they can still be similar enough to provide Apple-like user experience [to Apple's competitive disadvantage].
Based on the equally important reasons above, I predict that in these ongoing design/interface infringement battles, Apple is likely to have a lower hand against Samsung.