Learn English – Why “pastime” but not “passtime”

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?

Further details:

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'?}