First of all, originally the 738 wasn't offered with winglets at all. They were only offered 3-4 years ago for the first time. So any 738s manufactured before 1999 couldn't have them anyway, and any after that only if the airline specifically asked for them. New orders signed after 2000 could choose whether they want the option or not, I think. Some charter airlines were very eager (Hapag Lloyd, Air Berlin, South African - OK, not a charter, were among the first with winglets, I believe).
What does the issue boil down to? Initial purchasing costs, partly. But also the average duration of a flight. Winglets are most beneficial in cruise, therefore any flight that doesn't involve lengthy cruising doesn't really need them. Why would Ryanair want winglets on its fleet of 150 738s, for example, when most of their routes are shorter than 2 hours?
Anyway, that is my (hopefully) educated guess...