This question can be easily extended to any problem which can be solved by a library, security-related or not. This answer applies to libraries which implement any standardized functionality and isn't specific to SSL.
Not all software licenses are compatible with each other. When you want to use a GPL-licensed library, you need to license your whole project under GPL, which might not fit into your business strategy. Other organizations might reject anything but one specific licenses for ideological reasons. This leads to multiple solutions for the same problem which in practice only differ by license terms.
Even when a standard itself isn't patented, a certain method to implement that standard might be. In that case you either have to use the patented implementation method and pay the fee, move to a country where the patent isn't enforceable, or create a new implementation with a different method which works around the patent.
It often happens that two different development teams develop an implementation of the same technology independently of each other. When both solutions come to market, none wants to admit that their solution is inferior, so both will keep promoting and maintaining their solution.
Programming language preference
To make sure a library interfaces well with your program, it should preferably be written in the same programming language. When you have a C program, you would prefer a library written in C. When you use Java, you would like a Java implementation. When you use C#, you would rather want it implemented in .NET. It's not like it is impossible in either technology to interface with components written in the others, but it always requires some glue and duct-tape to get it to interface, which might impair performance, code readability and make the build toolchain more complicated.
Different projects have different priorities. Some want the fastest implementation possible. Some need a very low memory footprint. Some want the implementation with the best security track record (which can be measured in many different ways). The best library for one project isn't necessarily also the best for another. Having multiple options to choose from can be beneficial.