Because folks like G19s, but they don't have a great track record with 10-round mags. So a pistol built around a 10 round mag that works in a caliber that mirrors the performance of the legendary .45 American Communist Perforator makes more sense than carrying the G19 with maybe reliable neutered mags.
For me, the M&P45C makes more sense, or a LW Commander, but, that's just me. Actually, the sadly discontinued M&P45 2.0 Subcompact (4" barrel, G19 length grip, 8+1) made the most sense, but I don't know if they ever even hit the market.