I have thought about this too, Kyja. Just a suggestion, but could it have been that because there was no thought at the time of the sightings that he might have been doing anything suspect, there would be no reason to take a proper note of the details?
Isn't it quite a common experience to see a person then find it hard to recount exactly what they had on? I had coffee with my best friend last week; we sat next to each other talking for more than an hour. Can I tell you what she was wearing? No. I can only describe the overall effect she created.
The way I see it is that the specific clothing that the man was wearing simply didn't register, because there was no reason for it to. Only when details of this crime surfaced did people remember that they'd seen the man, because their memories of having seen an 'out of towner' had been jogged.
Perhaps there is a more suspicious reason but for now I personally would put it down to human memory. In general, a person retains memory of the most salient aspect of as situation, and that's often emotion-focused. So here we have the memory of the feeling or impression that 'he's not from around here', and it's the suitcase that springs to mind - indicating travel / being an outsider - not the actual clothes.