The reason is that U.S. Immigration laws only allocate 14% of annual quota to employment-based (EB) petition. The majority go to family-based (FB) petitions. Indian have a much higher EB/FB ratios, whereas Chinese have many more FB petitioners, e.g. cross-ethnicity marriages. Chinese also have one more eligible category than Indians---green card for asylees.
Ethnicities/nationalities like Mexicans have negligible skilled EB immigrants and have a much higher reliance on FB to immigrate a whole family to the U.S. (oftentimes derisively called out by Republicans as chain immigration), and thus they face longer FB backlog compared with Chinese FBs and Indian FBs. For example, as of today, the backlog for an Unmarried Child of an Mexican immigrant is 21 years, whereas that of Indians and Chinese are 7. (https://travel.state.gov/content/visas/en/law-and-policy/bul...) The priority date for a skilled Mexican immigrant programmer, who would be filing for either EB2 or EB3, is current (no backlog).
Since the system is heavily family-oriented, EBs often found themselves in very awkward positions as either the scape-goat for America's economic problems or distractions among partisan fights. A lot of things do not make sense. For example, the spouse of an EB petitioner does not count as "family-based" but takes up one more EB slot, and their school-age children, despite not being employed for work, are counted as "employment-based" immigrants. In addition, to offset the number of green cards given out to Chinese nationals after the 1989 Tian'anmen Square Massacre, a number of greencards are deducted from Chinese EB quota. Just making small changes to these two issues can ompletely eliminate Chinese backlog and greatly shorten Indian backlog. However, the politicians couldn't care any less about sensible solutions. While Democrats typically only pay the lip service, Republicans' common tactic is to hide their anti-immigration attempt behind a big banner of pro-skilled immigrants. The Rs either will try to trade the EBs with zeroing out refugee quota or slashing FBs in half, for example. They may have a good point (U.S. admits more refugees than employment-based immigrants annually), but touching that hot-button issue will never get Democratic votes, and it beginning to increasingly look like Republicans' political show vote.