You raise a valid point. While we don't have every part, we're constantly increasing the number of distributors and manufacturers who send us data. We have about 60 distributors on the site now which is about double what we had this time last year. We're always going to be getting better.
As for the part you mentioned, I didn't find a capacitor with those specs from TDK, but there are 12 alternatives here: http://bit.ly/hWnqhP
If you know of a supplier of the TDK part, let me know and I'll contact them.
That was just an example I pulled from nowhere, not something specific. I don't think TDK makes a 10uF/6.3V X7R cap (although I did notice the Octopart interface doesn't show temp spec/dielectric material as one of the specs that I can filter on... that's disappointing).
My point is just that in the course of normal use I want each one of these searches, be it for a capacitor or an apartment or a flight, to be very thorough. If I can't guarantee to myself that the search will cover 100% of the available inventory. I can guarantee that if I skip the nice frontend and go directly to the supplier.
FWIW, I picked the wrong one to pick on--Octopart has way better coverage than Hipmunk, Padmapper, or a multitude of other sites.
As for the part you mentioned, I didn't find a capacitor with those specs from TDK, but there are 12 alternatives here: http://bit.ly/hWnqhP
If you know of a supplier of the TDK part, let me know and I'll contact them.