Why is Saudi Arabia primarily buying weapons from the United States
when it could hedge its bets and buy a lot of Chinese and Russian
equipment as well? Is there a reason for this?
Mostly, it is a matter of historical inertia.
Historically, in the Camp David Accords of 1978, the U.S. bought peace for Israel, in the form at least, of tolerance of its existence and refraining from hostilities towards Israel, by providing military aid to Egypt.
A parallel deal had been in place with Saudi Arabia long before that time, in part to secure peace for Israel (under the rubric of "security" in the region), and in part to secure U.S. access to Saudi Arabian oil supplies:
In 1951, Washington and Riyadh signed the Mutual Defense Assistance
Agreement, which became the basis of US arms sales to the kingdom and
created a permanent US military training mission in Saudi Arabia.
Throughout the 1950s and 1960s, the relationship continued to expand,
as did the flow of US-made weapons to the kingdom.
Between 1950 and 1969, the US provided a total of $218m in foreign
military sales to Saudi Arabia, according to the US's Defense Security
Cooperation Agency (DSCA).
By the early 1970s, as Britain was pulling out of the Gulf - a move
that allowed Bahrain, Qatar, the United Arab Emirates and Oman to
declare independence - the US saw a chance to double-down on its own
interests in the region.
And between 1970 and 1972, the total value of US arms sales to Riyadh
skyrocketed from $14.8m to $296.3m, according to the DSCA.
That increase in weapons purchases coincided with an incredible boost
in oil revenues for the Saudi government, which pulled in $25.7bn in
1975 - up from $1.2bn in 1970.
Today, Saudi Arabia only supplies about 9 percent of all crude oil
imports to the US, as Washington focuses on domestic resources. US
weapons, however, continue to flow freely into the kingdom.
Between 2013 and 2017, Riyadh purchased about 18 percent of all US
arms sales, totalling around $9bn, according to a report by the
Stockholm International Peace Research Institute, which tracks weapons
The U.S. was interested in Saudi Arabian oil back in 1951, because a U.S. company got 50% of Saudi Arabian oil profits at the time:
The relationship between Saudi Arabia and the United States of America
was economically strengthened in 1933 when Standard Oil of California
was given the concession to explore Saudi Arabian lands for oil. The
subsidiary of this company, regarded as California Arabian Standard
Oil Company, later dubbed Saudi Aramco carried out a fruitful
exploration in 1938, finding oil for the first time. The relationship
between the two nations strengthened throughout the next decade,
establishing a full diplomatic relationship through a symbolic
acceptance of an American envoy in Saudi Arabia. . . . The trade
relationship between the United States of America and the Kingdom of
Saudi Arabia has long revolved around two central concepts: security
and oil. Throughout the next two decades, the 50s and 60s, relations
between the two nations grew significantly stronger. In 1950, Aramco
and Saudi Arabia agreed on a 50/50 profit distribution of the oil
discovered in Saudi Arabia.
After seven decades of receiving military aid from the U.S., Saudi Arabia fell into the inertia of continuing to make most of their arms purchases from U.S. firms (the U.S. wasn't going to provide these countries with military aid to purchase somebody else's weapons) and there are compatibility issues to some extent once you start down that road.
As another answer notes, this procurement inertia has finally started to fade after more than seven decades, but is still far from entirely gone.