Many Protestants reject transubstantiation for this main reason:
en.wikipedia.org/wiki/Eucharistic_adoration
While Catholics/Orthodox Christians believe that the consecrated host actually becomes the literal body and blood of the Lord, many Prostestants, especially Evangelicals, believe that transubstantiation is the height of idolatry, as it leads to Eucharistic adoration, which is the worship or latria (Note that this is not veneration or dulia/hyperdulia) of a piece of bread of which there is no chemical/scientific evidence that it actually becomes flesh or blood, much less that of the Lord.
Correct me if Iām wrong, but Eucharistic adoration does involve carrying the consecrated host in a monstrance in procession to be worshipped (ie latria) by the faithful, and that worship not only involves kneeling (genuflection) but also prostrations (lying flat on the ground).