The cloud-based software maker announced Wednesday it would buy Demandware, which provides software to design e-commerce websites, for about $2.8 billion.
"This was a very competitive deal. This was very hard," he told CNBC's "Squawk on the Street."
"We've never seen more deals and more things happening. We're not winning every deal. This is just a deal we were actually able to get done," he said.
Benioff said companies are positioning for growth next year, making it tough to close deals.
Salesforce's cash offer of $75 per share represents a 56.3 percent premium to Demandware's Tuesday closing.
Demandware's stock was up about 56 percent to nearly $75 midmorning Wednesday, while Salesforce was down slightly to $83.65.
The acquisition is Salesforce's largest to date, topping its $2.5 billion purchase of marketing software maker ExactTarget in 2013.
Benioff said the deal would allow Salesforce to offer clients one platform to manage customer information, which Salesforce currently handles, and operate e-commerce stores, which is Demandware's domain.
Demandware's clients include Lands' End, L'Oreal and Marks and Spencer Group.
The deal, slated to close in Salesforce's second quarter ending in July, is expected to increase the company's 2017 revenue by about $100 million to $120 million.
Salesforce had forecast fiscal 2017 revenue of $8.16 billion to $8.2 billion in May.
— Reuters contributed to this story.