A President's time is very valuable. Each minute is invested carefully, aiming for maximum benefit and efficiency.
Obviously, WEF-Davos was not, in the view of most presidents, worth their time. Either it didn't fit into their agenda priorities, or maybe they didn't know how to use the event for the benefit of the US, or maybe they didn't feel like flying to Davos in the dead of winter, or something else.
In a televised live broadcast I just finished watching a few minutes ago (the press has now been escorted out), President Trump is hosting a dinner in Davos for CEOs of 15 Europe-based global corporations. Based on each attendee thanking the President personally for the invitation, it appears that the invitations came directly from the White House.
He's using that event to thank each leader for their investments in the US and to gently, but publicly, pressure them to invest more. He specifically asked one attendee: "How much will you be investing [in the US]?" To others he was a bit less direct, but they all knew what the President wanted to know and answered accordingly.
So, for a US President with a strong background in business, he's using WEF-Davos to promote commerce in the US. This fits well with his "America First", "jobs jobs jobs" economic priorities.
Other President's had other priorities.