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USB supply voltage is 5V and maximum current allowed by specification is 500mA with mandatory integrated current limiting. Moveover this is only the case if you plug the device into proper USB port on the back of PC. Ports on front panel often have their power limited to some lower value. In portable devices things look even worse, because of constant struggle to save every microwatt. In portable devices drawing maximum current is possible but requires some twiddling with system settings (eg. power management in BIOS)
So you have 2.5W of power available at 5V. Leds have to be driven with current (as opposed to voltage) so preferred arrangement is series connection, which enables you to use only one current driver. There are special LED driving chips which are basically constant current switch-mode converters (you need some of step-up type). If you're doing LED lamp I assume that you want to use white LEDS. White LEDs are technically blue ones with phosphorous layer, which translates directly into about 4V of forward voltage drop. So to drive 10 leds you'd need a circuit that can deliver about 40V. This translates directly into ~62 mA of current assuming 100% effectivity of the converter.
You can of course use 2 branches of 10 leds each with separate driver but this forces you to decrease current to about 30mA per branch. I'd say that this can give you SOME light, but not worthy to call it a lamp.
Moreover 2 branches increase costs. Converter chips come in 2 flavours: with integrated switch and with external switch. First group is a bit more expensive but it doesn't need sxternal switching transistor. Other group tends to be really small fotprint (like sot23-6 style) and it needs discrete switch (at this power also typically sot23 or sot223 package). You also need at least one inductor per converter and some capacitors. Downside is that in many cases these capacitors are required to be low-ESR ceramic types of quite large capacitance, and extra capacitance translates into extra cost.
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