When you think of EMC buying RSA Security it may seem like a strategic fit for a storage company to provide security especially with Network Applicances (NetApp) having aggressively positioned itself in security around data encryption.
But it is the math that is most at play here- just as with most small enterprise software companies the cost of sales and marketing leaves RSA Security with only a 12% profit margin. Compare this with margins of larger enterprise software companies (with revenues of over $2 Billion) in the 30-40% range. On top of that EMC can sell security to many more customers worldwide that use its storage software.