Why is it that dynamics only really began to be frequently used from the Romantic period onward? Also what was the first recorded use of dynamics because I read on Wikipedia that it was first century ancient Greece but what would they have used as their dynamic markings?
A short history of dynamics in music:
If you look at the Baroque period, it is about conventions. Composers did not see the need to add dynamics and the players did not miss them. Not writing them did not imply there were no dynamics in performance.
Improvisation: Music was played quite freely. Improvisation around the theme was an important part of the performance. Figured bass was meant to be played differently each time. Same with the melodies, the first round could be played unadorned while the second round was embellished with trills and added ornamentations. In a way it was closer to jazz than symphony orchestra.
Fixed form: A lot of baroque music is in the shape of dances. People knew how to dance the gigue, the allemande, the tambourine and so on. As part of the dance the conventions told how to play the music. Sometimes strong, sometimes less.
Instruments: Some of the baroque instruments could only play at one volume or sometimes strong or weak, not in between. The harpsi-chord is one example. Several of the wind instruments were the same. The dynamic effects were, when possible, made by adding or removing instruments. Often you played the music on the available instruments, not as fixed in settings as in later times.
Function and venue: There were no record players at the time. Music was performed live. In homes, at dances, outdoors, in churches. And yes, sometimes on a stage. The music was interpreted and adjusted to these circumstances.
As time progressed, instruments as well as taste changed. Music was published and played far from its origins. And not the least, composers wanted to convey their intentions in a far more detailed way, hence the need for written dynamics.
The harpsicord could only do terraced dynamics (if it had multiple keyboards and/or ranks of strings), and this is often quoted as the reason for a general lack of dynamics.
But plenty of music was written for other instruments, and most stringed and wind instruments certainly COULD do dynamics! So I think we have to look beyond mechanical limitations.
I think it was just a way of musical thinking. Texture was more important than instrumentation. Many instruments were contrasted with few (or a solo). WHAT instruments they were was of secondary importance. Melodic ornamentation rather than additional volume added intensity to a repeated section. (Compare Constant Lambert's remark: "The whole trouble with a folk song is that once you have played it through there is nothing much you can do except play it over again and play it rather louder.")
Much of what's been said is true, except for the notion that the instruments were incapable of dynamics. Except for the harpsichord, all other instruments have a sizeable dynamic range, and could and can do dynamics. The composers however were often part of the ensemble and knew when to play loudly or softly, didn't have the time to make specific markings, or simply trusted in their fellow musicians' common sense. You see a similar lack of precision in food recipes of the time, not because they didn't have the means or didn't know how to vary amounts of particular ingredients, but because often they wrote down recipes for themselves, and they knew how much salt it needed, or anyone reading the recipe would be able to figure out how much salt they themselves wanted. The idea of terraced dynamics is an outdated attempt by mid-20th century musicians to explain the lack of sophisticated dynamic markings in baroque music, and is disproven in many 17th and 18th century sources.
In addition to the other answers, there has been a general evolution in Western musical notation from the less to the more specific. As first conceived, notation was not intended to convey all that a musician would have to know to play or sing a piece; it was a reminder to jog the memory of someone who had already heard it. Over the centuries, more and more information was added: pitches (rather than just indications where the melody went up or down), rhythm, tempo, instrumentation, and so forth. By the Baroque time, most of the information required to sightread an unknown piece was contained in the notation, but some things were still left up to convention and/or personal taste- for instance dynamics.