What possible reasons can there be that Punjabi Sikhs shy away from spreading Sikhism to other ethnic groups ?

Being a Punjabi Sikh myself, I had to be shaken out of my comfort zone and then pounded by wake up calls by dedicated non-punjabi sikhs that my version of sikhi is far from the original. They didn't say it but I came to realise it. I often termed myself as a lazy sikh because I had no discipline to get up early in the morning and meditate. Of course I do my Nitnem Daily, Rehras, Kirtan Sohila and Sukhmani Sahib but my mind wanders in four directions. I also engage in Kirtan sessions. Had taken amrit long long ago but ate Muslim Halal food from Malaysian Food Outlets on a regular basis. Never wore kechaira but a close copy of a ready-made underwear. Never wore a kirpan but kept unshorn hair throughout my life.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/essays-on-sikhism/1476-african-american-woman-how-we-preach.html

Another thing is the level of education gap between me and Americans; most of whom have a string of PhD s.

I can learn from them instead of teaching them. Although I do realise that I have to separate punjabi culture from sikhism, I keep falling back into the same entrapments which takes me a long time to realise that I acted no different than a punjabi in general.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=1476

But I do realise that Punjabi language is not as important as we have been led to believe in the past. Why? because Pakistani Muslims also speak the same language. So if given a choice to educate my children, I will choose to educate them in gurmukhi than punjabi.

Some will say the two are inseparable - NOT TRUE.