Growth strategy from dell
The reason is very simple.
Dell as invested massively in developing a large range of products that fits to multiple needs, while hp the same old concepts, blades hp dl360 and 380.... Bringing nothing but new processors mainly. Dell by the other side explored multiple concepts, in particular converged and modular approaches and product integration.
Final but not least, being responsible for a massive amount of hardware and ordering lots of kits, for the same equipment, dell provides cheaper prices. In terms of quality... They all use the same disks, cpus, ram... Is irrelevant, what changes is the box and interconnectivity. Another negative side on HP is the support that is largely poor, especially when it concerns to firmware, they have the obsession of creating custom drivers, that often are very buggy, Dell simply uses the ones that are done by the chip manufacture.
HP is falling behind... That is viewable in all fronts, there is lack of vision or too much influence from board of investors that are clueless about technology and just care about numbers.
Don't take me wrongly, because I work in a essentially HP house, with millions spent with them. But we started shutting because they don't provide answers for new challenges.