Coin collecting has been dominated over the last fifty years by third party grading issues to the point where few serious numismatists buy non-graded coins. Eager grading services have tried to push into philately hoping to enlarge their fee base. So far it hasn't worked. Some collectors have become enamored of graded stamps but most think that it is inconvenient and costly. There are several reasons why third party grading is popular for coins and why it has largely failed in philately, despite two separate, (one in the late 70's and the other a few years ago) highly promoted, and well funded grading service attempts to make it stick. Third party grading has created changes in numismatics that has made it very different from philately, with which it traditionally has shared many characteristics.
Third party grading encourages investors over collectors. As philately stands now, collectors usually spend a great deal of time in reading and study before committing large sums to their hobby. Third party grading allows ignorant investors (in the sense that they know little of the hobby) to come into a field in force, committing large sums on the basis of a supposed neutral third party grade (that such protection may be largely illusionary is borne out by the highly rated mortgage backed securities that people bought on the basis of third party grading in the financial sector and which caused the financial meltdown from which we are still recovering). Third party grading encourages trading as collectibles become more fungible. Most numismatists are constantly buying and selling while most philatelists mainly acquire. And further, many people are attracted to philately by the neatness of it. Numerous certificates and slabbing create storage and display problems.
There is a role for third party certification in stamps. Philately has many forgeries and repairs and, for valuable stamps, it is a good idea to have a third party certificate of genuineness. But most stamp collectors continue to feel that paying $25-$50 for a cumbersome third party grading certificate or slab with every stamp that they buy is something that would make their hobby an ordeal.