I'm going to assume it'd primarily be pragmatics. The amount of sperm storage and bureaucracy you'd have to prepare for every man in the IDF to potentially be able to store sperm for their (or their family's) personal retrieval at a later date would be off the charts, whereas there'll be significantly fewer people being pushy about zapping their dead son for grandkids (plus the limited window to achieving it further reduces the amount of sperm they'd need to process). Plus there might be a PR element - this way it's ensuring the brave soldiers can still father children from beyond the grave... the other way is more reminding conscripts that they might be going off to die or be maimed.
There's more widely the issue of
hotza’at zera levatalah (Onanism). It's not kosher to "waste seed" so highly observant Jews
struggle with family planning clinics and e.g. might need to bang their wife while wearing a sperm collection condom -

(with some Rabbis teaching that this is only acceptable if there's a little pinhole in it so some sperm could theoretically go in the wife), although they can do it the old fashioned way if the wife is niddah (on her period). This would be then further complicated if you were allowing sperm donations from all IDF soldiers, because what do you do with the donations from all the soldiers who return from the front completely intact and alive? I doubt Israel would want to maintain a national sperm repository for practically every man in the country, but flushing that sperm would be "wasting seed" since it's guaranteed not be used now, and there'd be objections from people who developed infertility issues at a later date who'd
had viable sperm destroyed by the state.
If you want to read some Rabbis sperging about the halakhic ethics of playing Five Nights at Feivel's,
this article has a variety of arguments about Levirate marriages and if dead mean can fulfil the Mitzvah of procreation.