As far as I can tell, this part is not available to you because no one in the community has invested the time to create an LDraw model for it. The model files for most libraries of 3D LEGO bricks are based on the LDraw models created freely and collaboratively by the community.
Your part should be on LDraw here, but it is not there, indicating that it has never been added to the library.
Let's take a look at the part:

Most LDraw parts are built from a number of common primitives, such as studs, boxes, and clips. For example, here's the entire definition of a 1x2 brick:
1 16 0 4 0 1 0 0 0 -5 0 0 0 1 stud3.dat
0 BFC INVERTNEXT
1 16 0 24 0 16 0 0 0 -20 0 0 0 6 box5.dat
4 16 20 24 10 16 24 6 -16 24 6 -20 24 10
4 16 -20 24 10 -16 24 6 -16 24 -6 -20 24 -10
4 16 -20 24 -10 -16 24 -6 16 24 -6 20 24 -10
4 16 20 24 -10 16 24 -6 16 24 6 20 24 10
1 16 0 24 0 20 0 0 0 -24 0 0 0 10 box5.dat
1 16 10 0 0 1 0 0 0 1 0 0 0 1 stud.dat
1 16 -10 0 0 1 0 0 0 1 0 0 0 1 stud.dat
0
It's easy to add parts that use mostly flat surfaces and common connection families. It is much more time consuming to add parts the include irregular curves and molded details. The above part would be time consuming to add, and would require the LDraw part author to have the physical part on hand, which likely contributes to why it hasn't been added.
For what it's worth, I saw that you also asked about this on the LDraw forums. That's a good idea, and hopefully one of the part authors will have this part on hand and be willing to add the model for you.