Why was the original definition of meter abandoned? 
The meter was originally intended to represent $1$ ten-millionth($10^{-7}$) of the distance from pole to equator of the Earth along a meridian of longitude.

The definition was later discarded. Now, what is the cause for this? What are the defects/flaws that make the definition unfit?
 A: The best standards can be independently and repeatably verified to high precision without excessive effort.  The meridional standard fails many of those tests.  While independent verification is possible, doing such a large survey to high precision requires considerable effort.  An amount that is beyond the capability of a laboratory for instance.  Besides the difficulty of the survey itself, the uncertainty in the current position of the meridian endpoints limits precision and changes in their location over time limits the repeatability of the standard.
The wikipedia article on the Archive Meter has some additional information.
A: There are a couple of major flaws.  One is that with this definition, it is very difficult to know the length of the meter accurately; to do so would require a precise measurement of a very long physical distance.  The same problem existed with the definition that superseded this one, which had the meter defined by a smaller physical strip; measuring a physical object is just not very accurate.
The second problem is that the Earth is not a sphere, or even an ellipsoid.  So there is no obvious or unique way to define the distance from the pole to the equator.  Along different meridians, the distances along the surfaces are different.  Moreover, those distances change over time as the Earth's surface changes.
My understanding is that it was realized quite early on that the first problem made the original definition unworkable for precision measurements.  So it may be better to think of the length of the meter as being inspired by the distance from the pole to equator, rather than rigorously defined by it.
A: The first problem was that they had to determine that distance. A temporary result based on a much shorter span of latitude was used as a stand-in, which in turn was used to create a prototype meter stick. People started using this preliminary result as the meter.
The French chartered expeditions to polar and equatorial regions to get a better handle on the true distance. This took years. By the time those expeditions returned and provided more accurate results, the prototype had already become established. The new, more improved result would have meant throwing out the meter which was already in place.
That original concept would eventually have been thrown out even if those expeditions had been performed on a more timely basis.  The concept of a meter based on the distance from the equator to the north pole along the meridian passing through Paris is a lousy concept. The distance varies from meridian to meridian, it only works for the meridian passing through Paris (by definition). It is not an easily reproducible standard. That concept (easily reproducible standards) was a later development in metrology.
A: The original definition of the meter was based on the length of  2-second pendulum.  You can look it up :-) .
The proposal to use Earth's size came a few years later and was not adopted until a century or so later, when people discovered that the pendulum times varied from location to location because of variation in local gravitational force. 
