I don't know why several were filmed at the El Rancho Vegas...it was the first one to open on the Strip, in 1941. They had the Western decor, which might be what some of the movies were looking for. Also, El Rancho Vegas might have been looking for the extra publicity of having movies made there...?

I'd saved this list when I was writing the article...you might find it interesting:

Movies Filmed in Las Vegas Area
Las Vegas is famous for many reasons, included in the long list or reasons would be the movie industry.� Over the years, Las Vegas have had many full length feature movies filmed in Las Vegas.
Following movies have segments or complete movies filmed in or around Las Vegas and/or Nevada highlights.� Movies listed also include movies for television, Cable TV as well as the big screen.� If you know of a movie we missed, please email us.�
LAS VEGAS MOVIES
1936 Boulder Dam
Filmed partly in Las Vegas, Fremont Street and Boulder Dam.
1941 Las Vegas Nights
Las Vegas footage.� Frank Sinatra's first movie.�
1944� Moon Over Las Vegas
Footage in and around Las Vegas.
1944 Sheriff of Las Vegas
Various footage of Las Vegas.�
1946 Heldorado
Times must have been tough in 1946, even spelling errors in the movie title plagued this movie.� Scenes highlight the Last Frontier Hotel.� Includes shots from the Fremont Street parade and Hoover Dam. Starring Roy Rogers.� Several shots from the annual Las Vegas rodeo, Helldorado.�
1946 Vacation In Reno
Even though the title says Reno, this film has many scenes shot in and around Las Vegas.
1949 The Lady Gambles
Partially filmed in Las Vegas, Hoover Dam and Lake Mead.� � Stars Barbara Stanwyck with a small part for Tony Curtis.�
1950 My Friend Irma Goes West
Filmed partially in Las Vegas, with highlight at the Flamingo pool.� Film debut of the Dean Martin/Jerry Lewis duo.�
1950 Dark City
The end of the movie is filmed in Las Vegas with highlights around McCarran Airport. Stars Charlton Heston in his first Hollywood film, and co-stars Jack Webb and Harry Morgan.
1951 Painting the Clouds with Sunshine
Filmed mostly in Las Vegas, with highlights of the Club Bingo (now the Sahara).� Ending of the movie features footage of the Helldorado parade.
1952 Sky Full of Moon
Filmed mostly in Las Vegas and various southern Nevada sites with highlights of the 1952 Helldorado parade.
1952� The Las Vegas Story
Filmed partially in Las Vegas, with highlights of local casinos and McCarran Field.� Stars Vincent Price and Jane Russell.�
1953 Sabre Jet
This Korean war film was filmed at Nellis Air Force Base just outside Las Vegas.�
1953 Split Second
Scenes from the Nevada nuclear test site, located north of Las Vegas approximately 100 miles.� Several other southern Nevada desert scenes.
1954� The Atomic Kid
Filmed at the nuclear test site in southern Nevada including local desert scenes.� Star, Mickey Rooney is exposed to an above-ground atomic bomb test.� At the "Casanova Club" in Las Vegas, his radioactivity alters every slot machine to pay off a jackpot.� � Wife of Rooney, Elaine Davis, plays his nurse.� Story by Blake Edwards.�