Interesting question.
Both are part 23 aeroplanes. Taking CS23, it has no requirement there to set a maximum operating ceiling - although there's nothing I know of that would stop a manufacturer declaring one if they see good reason.
Part 23 does refer to an absolute ceiling defined by zero rate of climb, and a service ceiling defined by 100fpm RoC.
I have a C172M manual to hand, and can't see anything about a ceiling in it at-all.
Diamond post a DA42 manual online at
http://www.diamondaircraft.com/libra...ev.%207%29.pdf (warning - takes a while to download, or did me anyhow). As you rightly say, it states:-
2.11 OPERATING ALTITUDE
The maximum operating altitude is 18,000 ft (5,486 m) pressure altitude.
With no explanation. The perf data in the same manual seems to show adequate climb performance at FL180 with both engines at MCP, so it's hard to see any good reason why this should not be permitted.
The main thing I can think of, and it's wild conjecture, is that somebody has decided that as it's unpressurised then FL180 is a safe ceiling in supplementary oxygen. That, arguably is rather pointless and over-conservative if it is the case.
The other possibility I suppose is some kind of engine limitation? Again however, I can't think of anything about the IO360 that would create a particular issue? On the other hand, there are high altitude ignition system mods available for the 360 which I don't think that the Diamond has. Maybe its playing safe for that reason?
G