Believe or not, but our original intention was to have EqualLogic the first storage we support with this functionality, and we even had to invest and buy the unit for our lab (Dell just would not give it to us for free, like all of our other storage partners do).
However, hands on testing in our labs showed that EQL's implementation of snapshots is extremely inefficient - both in terms of disk space EQL snapshots use (due to 16MB block size), and significant performance overhead running off snapshot chain. Because these limitations would prevent most customers from enabling periodic SAN snapshots, this kind of ruins the whole concept - so it made no sense to built our snapshot restore functionality around this particular SAN.
Now, with EQL being out of the picture, since we already had established relationship with HP storage team (over our HP StoreOnce partnership), it was natural to take a look at their production storage next. They provided us LeftHand units, and we found their snapshot implementation to be much better than in EQL. We also liked that they ship the same code as StoreVirtual VSA, providing all the same enterprise-class SAN functionality at much lower price point - which seemed to be a natural fit with big part of our existing customer base.