Arteries can become clogged over a period of many years, in some cases it can happen more rapidly.
Still, from what you describe, I guess that the reason that bypass surgery is advised is not that the LAD is too small.
The LAD is a big vessel and stents are placed in the LAD, even in cases where the LAD is fully blocked and has to be pried open first.
More likely is that you suffer from diffuse LAD stenosis, where the LAD is narrowed over a larger distance. Many stents would be needed to fully open up the LAD.
For more than one reason, a long series of stents in one artery is not optimal and bypass surgery is often preferred in diffuse artery disease.