TopBunk - the max zero fuel weight limit seems like the most plausible cause to me, however I would not think there would be a lot of space available for cargo with 148 pax with check-in luggage returning from vacation in a 738. I did see some boxes packed with seafood being loaded onto another plane I did get on to, but it seems that the weight of the additional cargo taken on could not have been very much mostly due to cargo hold volume limitations.
Aerocat - most of your reasons like obstacles, runway length, runway altitude, etc., would not make day-to-day differences in weight restriction. The day to day temprerature variations at SJO are remarkably minimal as well. Could bad weather at IAH (high winds, heavy rain, which was the case that week) make that much of a difference? My thinking was that a 738 on a 1300nm flight is going out with fuel tanks almost half-empty, therefore pretty light, even if an extra hour worth of fuel is taken on...
I just found it remarkable that an SJO-EWR flight on the same day using the same equipment would go out with a similar weight restriction, and EWR is not only about 600nm further, but also known for its ATC caused delays.