Why light was said to be faster than electromagnetic changes in ether? According to Wikipedia on the historical development of the Lorentz ether theory:

... Contrary to Clausius, who accepted that the electrons operate by actions at a distance, the electromagnetic field of the aether appears as a mediator between the electrons, and changes in this field can propagate not faster than the speed of light.

My question is: why can't changes in the supposed electromagnetic field of ether travel faster than the speed of light ?
Since light is an electromagnetic wave, shouldn't it have the same speed of propagation as that of the changes in the electromagnetic field ? Putting in consideration that by the time, light speed wasn't known to be the speed limit.
 A: Changing electric fields in Lorentz days as today were solutions of the Maxwell equations and therefore cannot propagate faster than light in that particular medium.
A: The fact that light has speed c can be shown from the Maxwell's equations. You can find that in any undergraduate textbook on Electrodynamics.
A: 
My question is: why can't changes in the supposed electromagnetic field of ether travel faster than the speed of light ?

The first clue is from the transformation of the wave equation.  If you try to transform it under a Galilean transformation, you will find it is not invariant.  For some waves, the folks of the time had some physical intuition as to why this would be so (e.g., sound waves carried/affected by wind).  However, for electromagnetic waves, this seemed to pose a problem since they don't really care much about the wind.
The famous Michelson–Morley experiment illustrated that electromagnetic radiation did not seem to care about the so called luminiferous aether, i.e., its speed of propagation does not change relative to different aether flows relative to Earth's motion about the sun.  There were several attempts to try and pigeon-hole things back into Galilean relativity like inserting a Lorentz contraction to explain away why the speed did not change.  This caused problems for many as it seemed to only apply to E&M but not other phenomena.  These problems and several others were the seeds that started to get Einstein curious and led, in some ways, to his three postulates of relativity.

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*Postulate of Relativity:  The idea of inertial reference frames and how physics shouldn't change just because you are in a moving reference frame (over simplification).

*Postulate of the constancy of the speed of light:  The speed of light is finite and independent of reference frame.

*Postulate of a universal limiting speed:  The speed of light is the upper bound on all physical entities.


Putting in consideration that by the time, light speed wasn't known to be the speed limit.

True, they did not know it was an actual upper bound but they also did not know of anything that moved faster.  Also, they could test the speed of light by the late 1800s so there was experimental evidence to support the statements.  In fact, Ole Rømer was able to measure the speed to within ~26% of the currently accepted value back in 1676.  James Bradley was able to get within a percent of the currently accepted value in 1729.  Léon Foucault was able to get within a percent of the currently accepted value using a different technique in 1862.

Since light is an electromagnetic wave, shouldn't it have the same speed of propagation as that of the changes in the electromagnetic field ?

Yes, it does.  Light propagating is a change in the electromagnetic field.  I guess I am a little confused by the phrasing of this particular question but I assume you are asking why the quote may imply that changes would propagate below the speed of light?  If so, I think you are inferring incorrectly.  I think the quote implies that the changes in the field cannot propagate faster the field itself, i.e., they are bound by the same propagation speed.
References

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*J.D. Jackson, Classical Electrodynamics, Third Edition, John Wiley & Sons, Inc., New York, NY, 1999.

