Because there are more buyers.
There were ~50 Tech IPOs in 2014. Will 2015 be a blockbuster year for tech IPOs?
There were hundreds of acquisitions, if you include the smallest ones, over 1,000 apparently: 2015 Will Be The Year Of The Tiny Tech Acquisition
It’s not apples-and-apples. There were more billion+ IPOs in tech than billion+ acquisitions, near as I can tell, in ’14. And there probably weren’t more than 100 or so $100m+ acquisitions.
But acquisitions are more common because there are more buyers, which is in part because they can buy at different stages and for different reasons than buyers in an IPO.