For me it's a combination of things:
1. Variety
2. Size- until I bought my first home a few years ago, I was an apartment dweller...
3. Cost--yep..I'm cheap! Actually, if I had the $$ for some of the top line stuff I'd be all over it...but sadly, I don't. I can find stuff I like in 1/72nd that doesn't break the bank
4. I'm still learning...until I feel more confident in my detailing abilities, I'm shying away from the larger kits. I've got a 1/48 B=25 that I'm itching to build, but I want to get a few skills polished up first...and those are being polished on my 1/72 Mig, Warhawk, Hellcat and so on.
I like all scales, and just recently built my first 1/72nd tank (Italeri King tiger) and loved it. I think I'm hooked on armor in 1/72nd now!