Are 3.5 inch SSD drives available? Why so many 2.5inch versions?

It is cheaper to produce just one version, instead of two. 2.5" variants can be used in laptops and desktop computers which makes them more flexible. The 3.5" versions could only be used in desktop computers.

The technology easily fits into a 2.5" case. There is no need to “artifically” produce a bigger case than necessary.

There are 3.5" SSDs available, but they are not common:

The 2.5" form factor presents a lot of benefits when compared to 3.5" ones: they are smaller, lighter and cheaper (require less materials to manufacture such as aluminum and plastic). Even though, 2.5" generally offers the same features such as speed and data capacity like the 3.5" ones.

Also, thinking of HDDs (this does not apply to SSDs), it also has increased energy efficiency (less power consumption):

So regarding costs and benefits, there are no reasons to keep SSDs (even HDDs) in a default 3.5" form factor, but if you really want to install a 2.5" SSD or HDD into your desktop, you could just use an 3.5" adapter such as this:

As @Marco already mentioned, it a simple design choice. If it fits in a 2.5" case, why make it bigger?

Even for desktops, smaller is better: Using an adapter, you can easily fit two 2.5" SDDs in a 3.5" slot.

That being said, there are some 3.5" SSDs. For example, 3.5" SATA III OCZ and 3.5" SATA II OCZ.