pastime n. An activity that occupies one's spare time pleasantly: Sailing is her favorite pastime. [TFD]
Etymonline says that it is from pass + time:
late 15c., passe tyme "recreation, diversion, amusement, sport," from pass (v.) + time (n.). Formed on model of Middle French passe-temps (15c.), from passe, imperative of passer "to pass" + temps "time."
So, why did one "s" drop (Did it drop? Maybe never dropped) or why not double "s"? (i.e. Why not "passtime"?) Is there any historical or orthographic reason?
I think pastime is often confused with past time and passtime. I'm not sure if passtime is a word but I see a significant amount of usage in written works [in Google Books]. (Is it an incorrect usage in every case? Can it be a dialectal spelling?)
Pass time means to spend time doing something. [Even, pastime was used as a verb in the past.]
It gets even more complicated when there are noun forms of passtime and pass time with different meanings.
I see the usage of "passtime" as the amount of time passed in technical contexts. Also "pass time" is used in this sense in specific contexts. For example:
In road transport, the time that elapses between the moment when the leading vehicle of a column passes a given point and the moment when the last vehicle passes the same point. [TFD]
Note: I'm aware that English spelling has a lot of irregularities but we are here to discuss the finer details of the language. I have a good case with details and I believe there might be hints from the history.
For example, a similar question: Why is "gauge" spelled with a 'u'?