Many synagogues sing Kel Odon and Mimkomcho on Shabbos morning. Why are these two the most commonly sung parts of the Shabbos morning Tefila?
Kel Odon is a פיוט (liturgical hymn) and hence would seem to be designed to be sung. This in line with other פיוטים that we sing - such as those in the High Holiday Prayers and the Zmiros on Shabbos.
As far as Mikomcha - I think that caught on mostly due to the catchy tunes associated with it. It is much less common to find a Chasidish minyanim that sing Mimkomcha (as they tent to shy away from "modern" tunes durring prayer), however in the Yeshivish/Modern world it is very common (S. Carlbach's influence perhaps?), (although there are plenty Chasidish places that do as well).