The dtilt is a poke, it isn't there to "pressure shields" you aren't gaining much by landing it on a shielded opponent, and it's not really pressure because it has virtually no safe follow-up pressure. If you define shield pressure as a single hit, then sure, by that definition it is shield pressure, but I'm thinking most everyone else considers shield pressure to be a string of hits, all relatively safe, that is done in order to weaken your opponents shield or to catch them at some point during the string of attacks with one of the hits, or perhaps a grab mixup during the pressure string.
If you think that you can sit around spamming d-tilt against the spacies (or most of the cast), even when spaced perfectly, then you will learn very fast that they will just start using short hopped aerials. The biggest problem with the d-tilt is that it doesn't cover aerial attacks, therefore, it's only safe on a grounded opponent. Compare this to Fox/Falco shield pressure, in which it is largely safe as long as they are in range to do so, don't overextend, and/or **** up tech.
Marth has no safe form of pressure strings like this, period. In fact, performing any aerial in neutral is a pretty bad idea, as you overly commit. Marth's greatest strength is his ability to counter damn near everything with his attacks due to his range and priority. By throwing out an aerial, you give up Marth's greatest strength and greatly limit his options. The only time where it is appropriate to really use aerials as pressure is if you have your opponent trapped in place, as in they are shielding under a platform, or are on top of a platform, and even that isn't really a long pressure string, more so an aerial or two, followed by a d-tilt, grab attempt, etc.
For examples of what I'm saying, watch Dr PP's Apex 2015 sets. Look for when he uses aerials, as it is almost never in neutral, and if he does them his opponent is either: in the air, on a platform, or stuck under a platform near a ledge/near a ledge.
It has everything to do with this discussion.
Sure, if you want to nitpick things down to each individual matchup and character specific things then yes, that is obviously a part of a character's overall difficulty, but I'm talking from a very macro-perspective because it's impossible for any of us to agree upon something that is so micro. On a macro level, people tend to play characters that match the style of play they prefer, and that does have a psychological affect on people. It's much easier to go with the tide than against it, and for some people, Marth's reliance on patient, semi-reactive play would be more difficult than Fox/Falco's barrage with safe pressure and rush-down style.