There are lots of words that have male and female forms, and usually there are alternate suffixes to the words which indicate the gender; for example, "waiter" vs. "waitress", "mister" vs. "mistress", etc. The one that has always puzzled me, though, is "widow" and "widower". Following the form of the previous examples, I understand "widower" for men--but why the form of the word with no suffix for women? Why isn't a woman called a "widowess"?
I suspect because the phrase was only needed for women and widower is a much later literary invention.
Widow had a lot of legal implications for property, titles and so on. If the survivor of a marriage was a woman things got complicated before women had many rights. If the survivor was a man in the middle ages it didn't really make much difference as he held all the property anyway.
A similar question came up about illegitimate girl children, there was no word because there was no legal need to consider them.
For the rest - English generally doesn't have many genders anymore and those that have survived are where it was necessary to know the actual sex. So for example "actress" once had rather more of a euphemism role (like the modern 'model actress whatever') — where knowing their sex is relatively important.
The term widower is obviously an extension of "widow":
mid-14c., extended from widow (Etymonline)
"Widow" is old enough to be credited to Middle English "widewe" and Old English "widuwe".
My guess for the term starting with women is the economical independence that men have been granted throughout history. The strength of polygyny over the rarer polyandry also implies that the death of a wife wouldn't have altered a man's married state due to his having more than one. The need for a male counterpart just wasn't as great.
Of note, I have rarely heard "widow" refer to a man who has lost his spouse but it seems ridiculously uncommon (and thus potential errors) and most of the definitions I saw specifically referred to women. The shift back to gender neutral language may effect this. Also, "widowed" is correct for both genders (as opposed to "widowered".)
Historically men would die before their spouses because of their involvement in war and there were not so many men predeceased by their wives hence the term widow was applied to women whose husbands have died. The term widower would have been used later in life when a need arose to distinguish between a man and woman whose spouse has died.
Widow essentially translates to "empty house" in one of webster's and the bible's opinions. This needing definition in and of itself, I would posit that the reference is to the fact that women had no standing as far as owning anything, they could not vote, they could not buy or sell property, they could not convey property directly to another person, they were almost considered as "Chattel" themselves. So, without a whole lot of research, this would be speaking a couple of things to me, one would be that the Husband of the lost wife would be expected to handle those few matters that the wife would have had standing in, as well as his own, essentially thereby performing the duties of the widow or you could say that he was considered the widower to that estate.. just a thought but it makes sense.