Because in the NFL or Major Leagues, they only mention usually what school people went too. So if Taysom went to Clemson how does that translate to having press for the Church? It doesn't it, but when people talk about Taysom and he played at BYU. I would venture to guess that 90% or more average fans that are not LDS would assume that he is LDS.
That is just the way it is. And most kids that go to big time schools like that don't go on missions.
For example if you did a poll of a average fan that doesn't know much about the LDS church and asked them if Bryce Harper is LDS I would guess that 70% or more would not know. But if you asked a average say basketball fan if Jimmer was LDS most people would assume so, just because he went to BYU.