Because the visas are distributed by lottery, and the largest number of applications are filed by Isfosys-type spammers. People with >$130k jobs in SV are being denied due to the $70k body shops spamming the lottery. If the spam goes away, those non-spam applications become a more reliable proposition.
I'm not saying it's morally right or wrong. I'm just saying the set of people that get H1B visas will change, and the shift will be toward higher-end jobs.
Why would it make a qualified American developer less attractive to Google?