Style difference definitely is a big factor in it, but just now I can name one huge improvement: form. The first drawing's anatomy looks wonky and off, especially in the chest and back legs (it's possible the back legs was style, but the chest, no). The newer one just has much, much better anatomy.
Second is the use of colour. The first has a very uninteresting colour palette, with little difference in saturation or value. The second, however, uses a whole range of values and saturations, making it much more visually interesting, from what I can see.
Thirdly, the actual skill required for the newer one is much greater than the skill required for the first. There's just sooo much more detail in the second, and that detail is done much better than the limited detail in the first.
So while you may not see it as an improvement due to personal tastes, the skill level has definitely jumped up a few notches from what I can tell