Because they are most often the key ingredients in shaving soaps - but not together. A soap is either glycerin based or tallow based.
Modern soaps (sort of speak) is made with glycerin, I think mostly because it is cheap and easy to use. This normally will give a softer soap.
What could be called "old" soaps would be made with tallow and this normally makes the soap hard.
But I'm sure some of the expert gents around here can give a better and more thourough explanation.
Tallow produces excellent shaving characteristics (but it's not so good as a cleaner) when it's saponified.
'Natural' soap contains a lot of glycerin--it's a byproduct of the saponification process. Commercial soapmakers skim it off and sell it separately.
This is part of the reason most bath soap makes bad shaving soap.
My guess is that good quality tallow soaps probably retain their natural glycerin, which increases their quality.
Usually tallow soaps are milled (run though a press) to make them hard, and they last longer. Together with good ingredients you get a superior, albeit expensive, soap.
So both tallow and glycerin are good, and they aren't mutually exclusive.