A better question is why the other spies you list didn't get life terms. Most on your list are obscure to the point that I couldn't find much information about their cases except at jonathanpollard.org. I did find this information about Marie Scranage where we learn that she cooperated with authorities to arrest another spy, her boyfriend Michael Soussoudis.
Were there any mitigating factors like that for Pollard? Isn't it the case that, rather than cooperating, he tried to flee arrest? What is the comparative damage done by Pollard's spying vs. Scranage and the others?
It's not clear that Pollard did much of any damage. Most of what was attributed to him at the time was actually the work of Aldritch Ames. 90% of what he was accused ot taking, he had no access to. What he did take, generally, were shipping records (which were his department) that appeared to be terrorist arms traffic.
John Loftus claims that this was part of Pollard's undoing: on information from Pollard, the Israelis tipped the Greeks to intercept what they believed to be an arms shipment to the PLO. Loftus claims that this was in fact a shipment of arms to Iran--the first shipment in the "Iran-Contra" affair. This of course suggests a reason that Bush Sr., at least, would want Pollard put away. Ironically, though, if this is true then it's Bush who should have been put away.