Certainly personal preference how you like to feel while riding. Strapless is a move nuanced balance, where straps allow you to man handle the foil more aggressively.
Personally there are a number of factors. Safety is a big one. I don't want to tweak a knee or ankle, and ride enough it would be inevitable. It also stops me from exploring big airs with a foil. Many love it but just two days ago I was out in nukin conditions and learned that just down the lake a veteran rider known for his amazing airs on foil died. Gave me a real shiver. That's the second time I've come back from a super high wind session to learn someone relatively local had died kiting the same day. Strapless riding with small kites is definitely a conscious choice for me. I have kids and I ride conditions big enough to make me nervous even after 20 years of windsports. I like to know my equipment choices and riding style improve my chances of survival.
In light winds, I really just like the freedom that strapless provides.
True to Purdyd's observations, I preferred to ride surfboards strapless too.