you may be right that Shaw is undervalued or TAA is overvalued and it's a good question - but I think this is a good example of the problem at looking at any stats in isolation, and especially end of year Divs totals when looking at value. It's important to drill down sometimes and put them in context.
Shaw won PB once this season (4p) and MB 6 times (33p). This sounds good (and indeed was good if you were holding!), but the one PB win was from the very opening day of the season vs Leicester, when there was only one other match on.
The MB wins were mainly in September, and the last one on 18 October. I can't remember exactly, but I suspect they were all linked to Jose Mourinho's treatment of him. Since Jose has gone this is no longer a story. Something like him being left out of the England squad ("Ha, ha - Man Utd's POTY not even good enough for England") did get some coverage but nowhere near troubling MB payouts. So with Jose gone it seems so has Shaw's realistic chances of any MB (unlike Pogba for eg), and therefore the vast majority of his 18/19 Divs wins.
Personally I think TAA's MB appeal tends to be a little exaggerated - he's won 2p once last June. But the perception seems to be as a "young English player at a big club" as you say, he is a MB candidate.
But looking at his PB wins this season, he picked up 4p at the start of the season too - but also won two treble match days in April and May for 5p each, which is the equivalent of 30p pre share split, when Shaw had all his wins.
TAA has a PB average of over 100 this season and has been consistently hitting high peaks - partly due to the fact he is one of Liverpool's main set piece takers, which is always a big factor in PB scoring. Shaw's average is 75 and he struggles to hit the peaks like TAA which can win the big money on Treble Match Days.
Apart from this, Liverpool are in the CL Final, and will be in the CL again next season - and he's also got a couple of England Nation League games coming up (if Sir Gareth decides to pick him over Trippier of course!)
In short, TAA owners have a lot to look forward to at the moment and people are willing to pay the premium - Shaw, not so much.