The reason Hitachi Ltd. was trying to sell HGST is simply that they are desperate to put themselves back in the black, they are eager to sell their volatile high-tech industry holdings in favor of more stable heavy industry, medicine, etc., and HGST is actually doing very well in recent quarters (consistent growth, good profit over several quarters, industry leading gross margin, growing market share, etc.). Since the HDD industry tends to have pretty steep ups and downs, it was imperative for Hitachi Ltd. to sell their storage division now (when they could get a good price for it) rather than later.
The ~$4.5b price they were able to get is very good, especially when you consider that the market capitalization of the much larger Seagate is only around 4b. If Hitachi Ltd. had instead tried to IPO their storage company, they would not get anywhere near as much as what WD was offering.
The only reason WD was willing to pay so much is for the immense value of eliminating a major competitor, and to give themselves a leg-up in the enterprise sector where they are not a big player (and Seagate is). Additionally, with only 2 major players remaining, it should be understood that there would be less competitive pressure and more potential to cooperate & coordinate pricing between the two major companies remaining.
At any rate, Chinese Ministry of Commerce essentially killed the deal earlier today, by "conditionally approving" the merger under terms so stringent that it would essentially be no merger at all (requiring HGST to be operated as an entirely independent business, retaining is current brand, manufacturing plants, independent finance, administration, R&D, with no sharing of company information between WD and HGST...all for a period of 2 years after which WD could only appeal to MOFCOM for a re-investigation and hope that they change their mind). Further, MOFCOM gives WD only 1 week to send them proposals for meeting such requirements.
Since there is a March 7 deadline for the deal to close (else either WD or Hitachi Ltd. can walk away or re-negotiate, and WD must pay ~$250m to Hitachi either way), this seems to be China's way of essentially saying "No". Frankly, it seems that given these highly strict conditions, WD has no real incentive to go through with the deal (especially for such a steep price). It will be interesting to see how they proceed.