We just didn't have a MOH or Best Man. The were both just bridesmaids/groomsmen. Why does one need a higher title?
I imagine this varies by social circle, but I have heard (at various times from various people) that the MOH is also expected to:
1. Serve as one of the witnesses to the wedding
2. Organize a shower
3. Assist in wedding dress shopping
4. Take notes at gift opening(s) of giver and gift, so the bride can more easily write thank-you notes later
5. Assist in planning any wedding or reception details (that the groom doesn't care about)
6. Help the bride with her dress when she needs to use the bathroom on The Day
7. Help with any wedding-related tasks like addressing and mailing invitations, making up favours, etc.
8. Give the bride a gift of lingerie for her honeymoon
I was unaware of any of these but item 1 when I was MOH for my older sister - I was seventeen at the time. I was quite mortified to discover that my cousin had planned a shower for my sister because Cousin thought I probably wasn't aware of item 2. The fact that this was true - I didn't know - was further mortifying. My sister told me then that all she wanted of me was to stand up with her, be her witness, and sing a solo at the wedding, and nothing else was necessary. (I was well aware of all three of those requests when I agreed, by the way.)
When I married, we didn't want a wedding party, and I didn't want to stir up any feelings by choosing anyone as a MOH anyway. Two of my friends helped me with wedding crafts. One of those two went dress-shopping with me (because she's the only person I know who loves shopping and who can keep me trying on clothes for more than twenty minutes on any shopping occasion), did my hair and makeup, and was my witness. In our thank-you speech at the end of the reception, I believe I said that she was essentially my MOH, but didn't get forced to wear an ugly dress today.
So, to finally answer your question: higher title is earned because they have (presumably) had more responsibilities.