Baptists believe that speaking in tongues, along with prophecy, and some of the other special gifts are not for today...they were just for back then in the Bible and that God never meant us to continue it into today. As an AG member, raised Baptist, I can say that I was missing something until I stepped into an AG church and learned about the Holy Spirit and speaking in tongues. While not essential, it is an awesome benefit. Why the Baptists seem to think there aren't those spiritual gifts today, I don't know.
Not all Baptsits believe the exact same thing. Some Baptists believe that when the Bible references "speaking in tongues" it should be translated as "other languages"and that it does not refer to random noises that nobody can translate.