The Sackbut is often called the forerunner of the Trombone or the "Primitive Trombone". Unfortunately, both these statements are rather confusing. The sackbut was certainly not primitive, it was the single most advanced brass instrument of the day, and remained so for hundreds of years. Compared to other instruments, the Trombone has changed less than any other member of the brass family, and the typical trombone shape is easily recognisable in instruments of any period. The important differences between the Sackbut and the Trombone are principally: (i) The addition of a venturi to center the sound, (ii) The fashion to favour a wider bore, which produces a fatter, more brassy timbre (Sackbuts have a more vocal quality than modern Trombones), and (iii) the ever increasing bell flare diameter, which amplifies the instrument's volume. Despite these changes, the basics of how this instrument works (the telescoping double slide) have remained the same since the mid 15th century.
"Sackbut" (in it's various forms) was a very English term, but even we English eventually adopted the more widespread Italian name "Trombone". In Italy the name of this instrument has always been "Trombone" and likewise in Germany where it is called a "Posaune".
The Trombone existed, in it's current shape and form, long before the development of many other wind instruments was complete. As early as 1468 the "trompette saicqueboute" was used to play music at the wedding of Charles the Bold and Margaret of York. which took place in Bruges. That was some 346 years before Heinrich Stoelzel applied his piston valves to the trumpet (and horn) and some 363 years before the emergence of the Hekel key system for Bassoon.
Three main factors led to the Trombone occupying it's position as Prince Among Brass: (1) The ease with which a skilled sackbuteer could play chromatically in any register (at a time in history when the trumpet [and later the horn] were limited to the tones of the harmonic series); (2) The method of changing from one note to another - using a double slide, allows the musician to play with perfect tuning at more or less any pitch, whereas instruments with finger holes are usually made to work optimally at just one pitch. This adaptability would be a huge asset when the pitch (of other instruments) might vary enormously from one location to another, or even within one establishment. The simplicity of a moving slide also allows for further adjustments to correct faulty tuning, or to choose between mean-tone and equal-temperament tuning without the need for complicated cross-fingering techniques; (3) The range of expression and dynamics available on this instrument allowed it to transcend the usual groupings of "loud" or "soft" instruments during the Renaissance period.
The trombone was held in high regard right from it's earliest days, as this quote from Tinctoris (circa 1487) demonstrates:
However, for the lowest contratenor parts, and often for any contratenor part, to the shawm players one adds brass players who play very harmoniously, upon the kind of tuba called ..trompone in Italy and sacqueboute in France. When all these instruments are employed together, it is called the loud music.
In the 17th century, Michael Praetorius described the sackbut as the:
wind instrument par excellence in concerted music of any kind.
The Sackbut's versatility was a key factor for it's supreme position. It could be played quietly enough to accompany solo voices, strings or flutes, yet the same instrument would accompany the shawms and cornetts in tower music and martial parades.
Even well into the 19th century, the Trombone still retained wide acclaim with many composers. For example:
In my opinion, the trombone is the true head of the family of wind instruments, which I have named the 'epic' one. It possesses nobility and grandeur to the highest degree; it has all the serious and powerful tones of sublime musical poetry, from religious, calm and imposing accents to savage, orgiastic outburst. Directed by the will of the master, the trombones can chant like a choir of priests, threaten, utter gloomy sighs, a mournful lament, or a bright hymn of glory; they can break forth into awe-inspiring cries and awaken the dead or doom the living with their fearful voices. (Hector Berlioz)
Click here to read what Wikipedia say about the Sackbut.