Glass has one definite advantage: it's neutral in flavour. Pewter, on the other hand, is not and adds its own particular taste to anything which is consumed from it. Which probably explains the unusual attachment some drinkers had to vessels of this sort. Particularly for drinking Porter.
"Sketches in London" by James Grant, 1838, pages 126 - 127.
This text implies that not only was glass in wide use in pubs, but that pewter pots were very low class. Not something any respectable person would be seen drinking from.