Beats me, too. Cat 5 is solid and both it and telephone cable are rated at TNV (>50V) whilst stranded alarm wire is only SELV (<30V). Alarm cable is much nicer to use, though, and the colours are prettier, too!
I'm also surprised that alarms rarely use screened/shielded cable.
When the application requires screened cable that is used, usually on higher end control panels not necessarily used by DiYers, even then it's not Cat5.
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