I was looking at a trike on the Utah Trikes site and one of its selling points was its longer lifespan due to its chromo frame. How widespread are frame cracks on aluminum framed recumbent trikes?

Extremely rare, more so than aluminum framed bents. Practically
speaking aluminum frames, well treated, are a life time frame
for the vast majority of owners. They have sharp limits on the
stress they can tolerate, but the average rider will never
approach this limit (assuming sub clydesdale class and no crashes).
Chrome moly is easier to fab, cheaper to make and the most
common frame material for bents as a class. Chrome moly is
going to add 3-6# to frame weight for a trike and is friendlier to
designers as the tube diameter can be noticeably smaller than
aluminum.

To say aluminum is lighter than chrome moly is making quite a generalization. Weight depends on a lot of things, material density, tube wall thickness. That said, the reason steel tends to last is simply because nobody can predict what kind of use any given frame will get. Maybe the aluminum is rated for a 250 lb rider but the 225 lb owner rides mostly on gravel MUPs. The greatest problem for bike designers is that the people who buy them can never be fully truthful about the amount of abuse they will heap on the bike after purchase.

:)ensen.