Since you brought this up...
As far as why don't all believers speak in tongues. - it could be for a number of reasons. but I think the #1 is that they are unlearned in this area because they have not heard about it or they have been taught it is not for today and thus it stops people from wanting to learn about it.
And God hath set some in the church, first apostles, secondarily prophets, thirdly teachers, after that miracles, then gifts of healings, helps, governments, diversities of tongues. Are all apostles? are all prophets? are all teachers? are all workers of miracles? Have all the gifts of healing? do all speak with tongues? do all interpret?
(1Co 12:28-30)
You say the #1 reason people don't speak in tongues is because 'they are unlearned in that area'?
Were the disciples 'taught' it at Pentecost when they were speaking in tongues?
Scripture seem to indicate that we each have different gifts. Do all prophecy? NO. Are all teachers? NO. Are all Apostles? NO.
This is the reason. God gives gifts as He sees fit.
Read 1Cor 12.