In summary, because they are seen as low-hanging fruit, at least the staunchly Hindu of them; India not being on friendly terms with China or Muslim countries makes this an easy play for Trump. FWIW, there's and NPR piece from Feb 2020.
Indian Americans, about half of whom are Hindus, are the second-largest immigrant group in the U.S. after Mexicans. In 2016, fewer than 20% of Indian Americans supported Donald Trump, but Republicans think they can get a bigger share this year. One reason - Indian Prime Minister Narendra Modi is a big Trump fan.
With Trump now set to visit Modi in India, that friendship will get even more attention. Trump has made no criticism of Modi's drive to give Hindus a favored place in India, for example, by promoting a citizenship law that treats Hindu and Muslim refugees differently. Democrats and some progressive Indian Americans have criticized Modi over that campaign, but more conservative American Hindus endorse it.
Politico (same month)
Another [ad] says, “America LOVES India,” and features a photo of Trump and Indian Prime Minister Narendra Modi together. “Our economies have never been better, and the United States is eager to build a strong partnership with India.”
A third features Modi alone and lists education policies the Trump administration backs, including tax breaks to support private- and religious-school scholarships.
[...] Still, Trump’s job approval among Indian Americans was only 28 percent in 2018, according to the Asian American Voter Survey, a poll of registered Asian American voters. About 66 percent of respondents disapproved of how the president was handling his job.
But keep in mind that those pieces were written before Trump shut off H1B visas which may stuff that actually matters to some Indians and Indian-Americans. Ironically some of these families have what Trump and some Republicans call "anchor children"
PTI managed to interview a Trump campaign strategist more recently (June) and he explained their play in terms of battleground states (and downplayed any effect of the visas freeze in view of the great and eternal friendship. YMMV.)
In an interview to PTI on Tuesday, Al Mason, co-chair of the Trump Victory Indian American Finance Committee, asserted that due to the aggressive outreach of Trump, there are indications of a sizable swing of the community away from the Democratic Party they have traditionally supported and voted for.
"More than 50 per cent of Indian-Americans in the battleground states, who have been voting for Democrats thus far, are crossing over to support President Trump, in his re-election bid," he said.
"These 50 per cent of the Indian-Americans (switching away from Democrats), which constitute 50,000 to 60,000 additional votes for President Trump in each of the battleground states, will make a big difference," Mason asserted.
In Michigan, Mason claimed, 45,000 of the 70,000 Indian-American voters who vote for Democrats, will switch sides in favour of Trump -- giving him a total of 65,000 votes. Similarly, in Florida, his study revealed that 50,000 of the 110,000 Indian-Americans who vote for Democrats will switch sides in November, he claimed.
According to Mason, 50,000 of the 200,000 voters in Texas, 40,000 of the 110,000 voters in Pennsylvania and 45,000 of the 100,000 voters in Virginia who vote for Democrats are expected to switch sides in favour of Trump in the election.
Political pundits have identified Michigan, Florida, Texas and Pennsylvania as the top key battleground states.
While Michigan has 120,000 potential Indian-American voters, Florida has 190,000, Texas 470,000 and Pennsylvania 170,000.
"Right from the start of his presidency, Trump has been the best friend of India and Indian-Americans. Therefore, they value the genuine friendship between President Trump and Prime Minister Narendra Modi -- giving Trump credit for the same," he said. Responding to a question, Mason exuded confidence that the recent temporary suspension of the H-1B visa is unlikely to have any impact on this swing.
"No amount of temporary suspension on H-1B, H-4 visas, which is most sought after among Indian IT professionals, will ever affect the existing great relation between India and the US," he said.