It is not readily apparent to me why this is so. A search for some of the traditional 'terms used in Internet searches for flushing out hard core geeks' (TM) (pmcray will know the ones I mean :-) ) and 'triathlon' yield some interesting web pages.
I assume that there may be some element of the 'geeky people tend to get into sport after school if not later in life and then, possibly because there are no unhappy memories of it, choose something not done at school' syndrome.
Alarmingly, I can see the appeal and if you had asked me 'so tell me Mr, Weasel, which sport would you like to be good at, though obviously given your lack of application to, well very much really, it is frankly highly unlikely?' then I would have probably had this at the top of my list. This would have been true before I made this observation consciously and therefore the answer would not be out of any desire to conform to the stereotype. I shall add it to the more quixotic end of my list of ambitions, for the upcoming new year's list on the lines of talking_sock's list for 2004 here.
(I feel I should explain that I didn't particularly enjoy sport at school but not in any dramatic 'ohmygodthishasf***edupmylife' sort of way. In my case it was not being picked last or almost last when people were lined up to choose teams in PE at school that discouraged me from sporting activity - I was, but I didn't give a shit, basically. The biggest influence I think was that none of my family were remotely into sport.)
(if you do a search on Google for 'sotic' you get, inter alia,
Sport On The Internet Consultancy
Special Operations Target Interdiction Course (WTF???)
Societe Technique de l'Indutrie Chimique
Società di Ortopedia e Traumatologia dell’Italia Centrale
I think I like the last one the most.)