Why does the escort look like Lenny's wife?

This isn't the first time we see the call girl. There's an earlier scene [in the movie, later in Leonard's real time-line] where we see his reaction to waking in the room, then finding her in the bathroom. This scene has no direct sequel at all.

As with everything in this movie, we are never told 'why' anything happens, but we later see him book her visit and instruct her to set the objects in the room, with the instruction to wait until he's asleep, go to the bathroom & slam the door. This is the immediate precursor to the earlier scene.

The paired scenes give us his perception of the events of the night of the attack, but the two characters actually look nothing like each other - one is blonde, the other dark.
We see other flashbacks throughout the movie, in lesser detail, often in short bursts as the memory returns briefly to Leonard whilst he is doing something else. As he burns his wife's book & hairbrush we see glimpses of of him nipping her thigh. In other flashbacks to this same event, we see an insulin injection. Which is true?
We actually see many versions of the flashback to the original attack, but we are never shown a single 'true' picture, only glimpses of different versions of events.

We are not told why, but as we garner more information about the world as Leonard sees it vs the reality we eventually manage to build up, this adds weight to Leonard's denial of facts as they happened. He only accepts facts he chooses to believe and actively destroys others [the burning of the photographs; the intentional mis-construction of the license plate data on Teddy's car etc].

We eventually learn that his wife did not die in the attack, but from an insulin overdose he attributes to Sammy Jankiss. We are given occasional glimpses that the 'Sammy' he sees in the flash-back scenes is actually Leonard himself.

This leaves us with a conclusion - maybe or maybe not actually true, as with many other details and motives displayed - that he is attempting to reinforce the beleif that his wife died as a result of the attack and not from a later overdose. There are flashes of her blinking under the plastic sheet.
We are left with the conclusion after he murders Teddy that he perpetuates his own mythology, leading him to actually become a serial killer, constantly searching for a new John G.

There are many treatments & interpretations of this online. This youtube video gives a good overview of the chronologic order of the main points, but doesn't include this event. IMDB's FAQ covers a huge range of topics. One thing is certain about this movie - it is very open to interpretation.

It's been years since I watched Memento and I don't actually remember the scene in question, but the implication here is pretty clear. Lenny misses his wife, and is trying to replicate the experience of having her around by hiring an escort who looks like her, and having her do things that his wife would once have done when she was alive.