A teaching language
Pascal started its life as a programming language for teaching. It was carefully crafted to get people started and gently tugg them onto the right way: structured programming. And the language as such if compared to "Your-father's-C" is much gentler to the novice than C. I think its actually quite amazing that Pascal caught on as it did. I think it is worthwhile to have a look at Algol 68 http://en.wikipedia.org/wiki/ALGOL_68 to get an idea how other languages at the time looked like that were considered to be advanced.
The original Pascal had to be extended in order to "scale" for greater projects (i.e. no notion of modules/units, etc.). Implementations extended Pascal to acommodate the needs of application development. As mentioned: by Philip this lead to fragmentation.
While fragmentation happend to C as well, the real issue with Pascal is, that it was never unified in the same way as C has been. Niklaus Wirth, the creator of Pascal started to develop successor languages (Modula, Oberon) which was probably not the best marketing choice.
Still with Delphi, Pascal had retained a lot of popularity throughout the years, especially as a better alternative to Visual Basic 6. Over the time however support faded and now I think one can only speculate. I think that people considered it on a "professional" scale somewhere between C (and emerging C++) and Visual Basic. With Java and .NET emerging the whole programming language landscape shifted. Lots of projects were probably migrated to Java and the .NET platform eventually and the community lost its drive. That might be the real issue, once lots people "leave the ship" it gets hard for a platform to stay relevant.
Pascal was not so bad after all as some people remember it. Pascal compilers were really fast back then, because Wirth had done a really fine job at grammar definition, it is easily parseable.
Aging with grace
In the long run Pascal has aged with grace before disappearing from stage. It does look outdated from today's perspective but overall it has been a success story.
A lot of comparisons about its success highlight things that it lacks in comparison to C, however I feel like that with new high level languages emerging, it was actually too close to C, and the world was not big enough for both of them. Today, there is not viable alternative to C/C++ while scripting languages are created by the dozens.